Glow plugs 101 ***Ver. 2.0***

RussR

New member
Joined
Nov 22, 2019
Location
North Pole, AK
TDI
2004 Jetta TDI
Yes, very hard starting. Just seems as if the glow plugs aren’t staying on long enough for proper preheat. I’d look at other possible issue if I knew that the glow plugs were working correctly. -2 degrees and only 1-2 seconds just seems entirely inadequate.
 

NH2013Jetta

Member
Joined
Sep 1, 2019
Location
Seacoast NH
TDI
2013 Jetta Sedan TDI
Glow plug issues with 2013 TDI

Hi gang,
For the last six months (maybe more) I’ve been playing tag with a pair of trouble codes related to my Jetta’s glow plugs.one is code P0674 (glow plug heater cylinder four) and code P06C8 (Computer and Auxiliary Outputs). The dealer hasn’t exactly been a fountain of help just advising me to let the car warm up longer on the key before starting,
But the day of reckoning approaches because come June I need to go through state inspection and by definition any fault Code is an immediate failure.
So do I just purchase all the glow plugs and go for it? Do I worry about the wiring harnesses? Where can I get a harness for my TDI? Everything I have seen so far on this thread is earlier models. How much stays the same and what changes?
Thanks for any clues you can share.
FYI my OBD-II scanner is an older Action, so old it has a serial port. But it still works.
 

BobnOH

not-a-mechanic
Joined
May 29, 2004
Location
central Ohio
TDI
New Beetle 2003 manual
Can you try a different dealer (warranty?)? The codes indicate a possible relay or wire/connect issue. But you can start by testing the plugs, easy enough. Unplug the boots. Check resistance (ohms) on each plug right in place. Though it's a bit trickier, you can then check volts in each boot.
Disclaimer- Don't know the 2013, but I'm pretty sure the glow plug system is similar to all modern TDIs.
 

OlallaTDI

Member
Joined
Jan 2, 2020
Location
Olalla, WA
TDI
2005 VW Golf TDI Automatic // 2006 VW Jetta TDI Manual
I've read a ton of pages in this thread but don't see a specific answer to my question. In my 2005 Golf BEW I pulled out N10591609 glow plugs which are the ceramic 7v plugs. I assume that means my ECU was flashed when the 7v plugs were put in. Whenever I go to order new plugs, they keep giving me the 5v steel plugs (N10591607) or some variation of that. This time I went to the VW dealer with my VIN # and they still came up with the 5v plugs. Can I just run the 5v's or should I return these and get the 7v's? It has always been a good starting car, no matter the weather.
 

Tdijarhead

Top Post Dawg
Joined
Nov 10, 2013
Location
Lawrenceville PA
TDI
2003 TDI Jetta Daughters Car, 2001 TDI Beetle, Wife’s car, 2005 Golf TDI Mine, all 5 spds

They have a 4 pack set at a bit of a savings also.
 

Midwesthick

Veteran Member
Joined
Oct 26, 2011
Location
Denver
TDI
A3, GSW, 02-Golf
Just bought a set of Bosch glow plugs and that little tool(gimmicky I’m sure but peace of mind) from IDParts. Keep getting P0674 code every other day or so.

I have a FrostHeater installed that I use daily now so that’s been nice. Just time to replace those old things. Previous owner installed a new harness already.
 

Elektricar

New member
Joined
Dec 19, 2020
Location
Croatia
TDI
Audi 80 B4 1.9tdi
Hi guys, greetings from Croatia.

I have rough start issue, and rough idle for a few seconds.The problem is no power on glow plugs, they are ok, 1.2ohms on each. Relay checked(relay clicks after few seconds) working fine, all fuses are in working condinion. Do you have any idea?
 

Dan Grn

New member
Joined
Jan 15, 2021
Location
Brasov, Romania
TDI
Sharan 1.9 TDI
Does anyone know where the relay and safety for incandescent spark plugs is? For a 1999 Sharan 1.9 TDI

No electricity reaches the glow plugs.
The temperature indicates it correctly, on the instr. cluster it indicates the spark plug sign kept lit.

If I put a wire from the glow plugs to the + battery, it starts immediately.

Otherwise it doesn't start in the cold.

I went to the service, but they didn't know what the relay and fuse was.
Unfortunately it is from 1999 and only after 2002 the relays were marked with 103 and 102.

Images:


 

Cbub

New member
Joined
Aug 18, 2021
Location
Dublin
TDI
2000 jetta
Well, its been over a year since my original glow plugs 101 thread. I just got the code back so I had to do the testing procedure again and thought I would redo my How to with new pics & info. So here goes.

The glow plug codes are a popular code to be thrown on our cars. Several versions of the DTC are

16764 / P0380 code (Glow plug/heater circuit electrical fault (Q6))
17055 / P0671 code (Cylinder 1 Glow plug circuit (Q10))
17056 / P0672 code (Cylinder 2 Glow plug circuit (Q11))
17057 / P0673 code (Cylinder 3 Glow plug circuit (Q12))
17058 / P0674 code (Cylinder 4 Glow plug circuit (Q13))

The first code is for 2001 and older cars. They have a 2 wire glow plug harness, so just throw a generic code for the glow plugs.

The other codes are just examples of codes you might find on a 2002 or newer car. They have a 4 wire harness, so the ECM can specify the problem plug.But the trouble shooting procedure is the same for all codes.

NOTE: I have tested a few 2002's recently for glow plug problems. I don't know if this is the case for all of them, but the ones I tested have the cylinder order backwards. For example, if the code specifies a bad plug at cylinder # 1, then cyl # 4 may be bad. The last car I tested had 2 bad plugs according to the codes. Cylinders 1 & 2 were indicated as being bad, but actually it was plugs 3 & 4 that were bad. So don't always trust the codes. Check for yourself with the multimeter before replacing any plugs.

Now why was a code thrown? The ECM measures the resistance accross the harness wires. If it finds an imbalance in the resistance between the wires, It thinks there is a bad plug since a bad plug will have high or infinate resistance. But the big problem is that there are other factors that can cause the resistance deviation other than a bad plug. A bad relay, corroded or oxidised harness, a bad ground or a blown fuse can all cause the code to appear. So I will go step by step through each of the possibilities so you can test and rule out or confirm the actual cause.

The first step I suggest is to check the plugs themselves. Afterall, they are the intended culprit that should light the MIL. What you need to do is remove the engine cover and locate the glow plug harness. You will need a 10mm socket & a digital multimeter for this step.

Here is the harness:


To remove it, just hook your finger under each terminal and pull. they snap on & off like spark plug boots. Once it is off, just rotate it out of the way so you can expose your plugs:


Then get your digital multimeter (available at any auto parts store or Wal-mart for less than $20) and set it to the lowest ohm setting possible (usually 200). Then place one probe to a good, clean ground and the other probe to the tip of the glow plug:


Do this for each plug and take note of the resistance value of each plug. They should all be between 0.5 and 1.5 ohms. It doesn't matter what the readings are, as long as they are close to eachother (within 0.2 - 0.3 ohms). If you have a plug that reads high or infinate, then you have found the bad plug and it needs to be replaced.

EDIT: I have measured sets of plugs that have had a 0.1 ohm difference with no code being thrown. But one member who contacted me had a code thrown with a resistance of 0.6 ohms on 3 plugs and 1.1 ohms on one plug. This was enough of a variation to throw the code. So even though we do not know the exact tollerance difference, its somewhere between 0.2 and 0.5 ohms.

Replacing plugs is very easy. Its like changing a spark plug. You need a 10mm deep socket and an extension. Many reccomend a 1/4" drive socket to allow the extension to get around the injector lines, but I have done them with 3/8" drive sockets too. Or, if you have a wobble head extension, even better. Just spin out the old one & spin in the new one:


I strongly suggest putting in the new ones by hand. You want to make sure it spins freely before using the ratchet on it to make sure it is not cross threaded. start it by hand and spin it in at least 3 turns to make sure it doesn't bind and then finish withthe ratchet. Tighten to 11 ft/lb. If you don't have a tourque wrench, no problem. Just make them snug, but not tight. Grip the ratchet with the head in your palm so you can't use the leverage of the handle and tighten it that way. That should be close to the correct tourque without overtighteneing.

FYI, There are also 3 coolant glow plugs on the manual transmission cars. These are 12mm plugs as opposed to the 10mm ones used for starting the engine. The purpose of the 3 coolant glow plugs is to assist in heating the coolant. They are not necessary for starting and are NOT MONITORED by the ECM. So there is no need to check them to solve any codes. But it can't hurt to check them anyway while you are under the hood if you want to make sure they are OK. I have tested many of these coolant glow plugs and have never found a bad one. I suspect that they last much longer since they are not exposed to the same harsh environment of the combustion chamber like the engine glow plugs.

If your plugs all checked out OK, then you have another problem. It is likely your harness, but since there is no way to test the harness, we have to move on to other steps to rule them out. If all other tests check out, then that only leaves the harness, so we will address that last.

The next thing to check is the relay. To check the relay, you need to see if your harness is getting battery voltage. In order to do this, you need to unplug the coolant temp sensor. This will trick the ECM into thinking its really cold out and activate the glow plugs for about 20 seconds. This will give you enough time to turn on the ignition, run to the front of the car and perform the test. Here is the coolant temp sensor:


It is located in the housing below and behind the 3 coolant glow plugs(manual transmission cars). It is difficult to see, so you may have to feel for it. It unplugs in a similar way the MAF unplugs, just a smaller plug. You need to push on the tab with your thumb to release the clip while wiggling the plug off . Practice on the MAF if you want to get a hang of it.

Once it is unplugged, set up your multimeter to measure voltage. You need to set it to 20V on the DC side of the meter. Turn on the ignition (do not start). Then find a good ground and place the other probe into the end of the harness terminal like this:


You should get around 12V at each finger of the harness. The glow time is only about 20 seconds, so you may have to repeat the procedure to get all four if your not fast enough. Once the glow time is up, the voltage will drop, but it will still show a small votage at the harness terminals:


I'm not sure why there is still voltage, but it is normal. If you do not get 12V at any time, then your relay is not completing the circuit and needs to be replaced. The relay is an expensive part (around $125), but fortunately, it is not often the cause of the code. I have only tested one system out of dozens that I have tested that ended being a relay problem.

So if the relay checked out ok, then you need to check the fuse under the battery. This was why my MIL came on by the way. Until recently, I was driving around without a battery cover and the fuses on top of the battery were exposed to the elements. They became oxidised & corroded. Two of the big fuses were not conducting current between the two posts. I tested them by placing the multimeter on each post to see if there was voltage to both sides of each link:


In my case, I got no reading on the far side of 2 of the links. The glow plug fuse is the second from the right. I removed the fuses, cleaned them with sandpaper and reinstalled them. I also used some Deoxit D5 to help enhance the contact & remove the oxidation. If your fuses are clean looking, I wouldn't worry about it, but it can't hurt to check. Of course, if one is blown, then replace it.

So now you have checked everything you can check. If you haven't found any problems yet, then you likely have a bad harness. There is nothing wrong with the integrity of the harness, its just a bit of corrosion or oxidation in the terminals that is causing poor contact, resulting in the high resistance that is making the ECM think its a bad plug. Most of the time, replacement of the harness is not necessary. It can be treated with the product I mentioned earlier, Deoxit D5. I have used it on many cars with great success. Here is a link to a discussion on this product.

In some cases, the harness may be too badly damaged to be treated. This would only be the case if it was left untreated for any length of time. I had a realy bad harness when I bough my car, so I replaced mine. Here is what I found when I opened it up:


As you can see, the oxidation on the two center liners was quite evident. They are white in color instead of shiny like the outer ones. The poor contact created by the oxidation caused arcing when the voltage was trying to complete the circuit. The arcing caused those black scorch marks. This obviously caused a poor connection which resulted in the high resistance, causing the code to be thrown. This harness caused my CEL to come on many times. Once cleared, it would always return. Sometimes it would stay off for 100 km of driving and sometimes it would come back within minutes. After I changed the harness last December, I have not had a glow plug fault now for almost 18 months and 60k km. But as I said earlier, don't consider replacing it until you try to treat it. No point cutting out the original one and splicing in a new one if you don't have to. Treating it is cheaper, easier & less invasive that replacing it. But if you do feel you need to replace it, here is how I did mine:

First, cut the old harness off with wire cutters. To make it easier to splice in the new one, cut the wires as close to the harness as possible so you are left with longer pieces to work with. In my case, since I was redoing my connection just for the purpose of taking pictures, I cut mine at the old joint:


The new harness (two wire version) comes with spade connectors on the endes of the leads. Just cut them off since there is nothing to connect them to anyway. Then strip about 1/4 inch off each lead:


Then crimp on some butt connectors to one set of wires. I use uninsulated ones since I can get a better crimp with them. They will also look nicer (less bulky) under the heat shrink tubing. I got them from Radio Shack:


Then I slid on a couple of pieces of heat shrink tubing and crimped the other two wires to complete the splice:


I then used a heat gun to shrink the tubing to seal the joint:


I used heat shring tubing with a glue lining. When it shrinks, the heat melts the glue and it makes a water tight seal around the joint. I bought it at a tractor trailer repair shop. They use it for wiring trailers since the wires are exposed to the elemnts (rain, snow & salt). They are very good. You will also find it easier to do the crimping if you remove the vacuum ball and set it off to the side. There is more room to get the crimping pliers in there.

If you have a 2002 or newer car, then your harness has a long set of wires and can be plugged into the main harness. But it is somewhere in the main loom and removal of the air filter box and possibly the battery too to route the wires the same as the original harness. It can be quite a pain in the butt to replace. A good reason to try to treat it rather than replace it.

This will give you an idea of how to test & diagnose your glow plug fault and save yourself an expensive visit to the dealer. I have read of quotes between $500-700 to fix these codes. The plugs are available from www.tdiparts.com for $80 / set shipped and Deoxit D5 is available at lots of places online. Just do a google search for it. BTW, you need the 10mm plugs for the engine and the coolant plugs are 12mm.

Good luck and hope you can get rid of that pesky code for good with this procedure.

EDIT: TDI Allen did a writeup on how to change the 4 wire glow plug harness if you decide to change yours. You will see that it is quite involved, so that is why I suggest treating you harness before considering replacing it. Here is the link
Thanks I had a bad wire connection on the two wire.
 

bighair

Member
Joined
Dec 14, 2017
Location
Moncton
TDI
MK4 BEW
Hey guys, Sorry to revive an old thread yet again! I've gone through this thread, TONS of info. Here is my situation. I got the car in peices and put it together and it didn't have any plugs in it so I had really no base line. Dealer won't confirm the recall info for me with the VIN.

The issue I am having is when testing at the harness, I get 11V for 1 or 2 seconds then drops to 5V for the remaining time before dropping out.. Which reading should I be using to select my plugs? the 11v or the 5v? I didn't pull the temp sensor to test because I had someone in the car cycling the iginition and I was at the meter so i had time to get the readings. I have 5v plugs in there now and GP#1 and I have to replace s bad hence my reason for revisiting. I just want to make sure I have the correct voltage as I have been having cold start issues since the beginning, even after putting these 5V plugs in before. I ended up just installing an inline heater and if staying longer then 4hours somewhere try to park close to a plug lol.

If the readings indicate the 5V plugs, why in my case is the initial voltage 11V?

Tks, just trying to get a better understanding..
 

aprilfrankenstein

New member
Joined
Jul 5, 2020
Location
Denver, Colorado
TDI
2003 VW Jetta TDI
Thank you WINGNUT! I went through every single step and finally FINALLY, sanding down my fuses and spraying with Deoxit is what was the fix. I would suggest to others to start there. My 2003 jetta is starting up great with no more stinky fuel odor and FAR less white smoke- almost none. Thank you so much. So glad I didn't let the mechanics replace my glowplugs and relay like they thought was the issue. Saved me $400. If you haven't already you MUST invest in a muiltimeter! Got one at Home Depot for under $40. THANKS!
 

Pprey

New member
Joined
Apr 28, 2021
Location
Spain
TDI
2015 VW Caddy
I have a strange glow plug fault in my 2014 Caddy 1.6tdi. I am getting P071 and P0672 on new plugs. If I turn on the ignition for 10 secs and then turn it off, then turn it on again and start the engine - no fault. If I just turn it on and start the engine then I get the fault. The other odd thing is that this happens when it is quite cold (8ºc). If I don´t start it until the temp has risen to 10 - 12ºc I always get the fault. Any ideas? The ne plugs are Bosch.
 

swordpolo

New member
Joined
Jun 4, 2022
Location
İstanbul
TDI
Polo CayB
Hello.. my car was CayB 1.6t.. my problem is that although the air temperature is 25 degrees, my car starts hard at the first start.. glow plugs do not engage.. but if I unplug the fuel temperature sensor, it is activated and the car starts easily
 

DFDJ

Well-known member
Joined
Jul 3, 2001
Location
Enfield, NH
TDI
2015 Golf Sportswagon SEL, 01 Jetta Baltic Green Sold at 277,500 mi,
I've got a long first crank of the day problem (posted on the long crank thread) which only seems to happen when weather is warm. The problem started this summer (80k on a 2015 GSW) and disappeared when nights got cool, came back after a few warm nights and disappeared again with cool nights. I've asked the question "Can the temp at which the GPs come on be adjusted with a vag-com?" because that seems to be a logical work-around to what appears to be a problem without a know solution. (Cam shaft sensor/adjuster, battery and fuel lift pump have all been suggested as fixes that work sometimes or for only a short period of time.) I'm not one to throw new parts at a problem unless they are proven to need replacement, so, since long cranks after warm nights are an annoyance that I can live with I don't want to go that route.
So, it is possible to raise the temp at which the GPs come on before the car cranks? Any thoughts as to how to make this adjustment would be appreciated!
 

Full Circle

New member
Joined
Jan 24, 2023
Location
Central TX
TDI
2001 Golf GL 2DR HB 5spd manual 93 Ford F250 7.3 IDI NA / sold 2009 VW SW / 2010 GOLF / 2014 BMW 323D
Thank You Wingnut just set this code today scanned with vagcom went to Ross tech wiki and they have a link to your post. Went to test my glow plugs and my harness crumbled into pieces. Will still test to be sure in the meantime time to order a harness, thanks again.
 

hskrdu

Top Post Dawg
Joined
Oct 17, 2003
Location
Maryland and New England
TDI
2003 Golf GLS 4D 5M, 2015 GSW SE 6M
Sorry, I have not read all sixty pages to get here.
This thread is about glow plugs. Ask the mods to move your post to the correct forum where it will get more replies: https://forums.tdiclub.com/index.php?forums/vw-mkvi-a6-jetta-sedan-2011.25/

Or delete your post and ask your question as the first post in a new thread in the forum above. If the car was running fine prior to the water pump leaking, you might want to include your timing belt replacement procedure.
 
Last edited:

hskrdu

Top Post Dawg
Joined
Oct 17, 2003
Location
Maryland and New England
TDI
2003 Golf GLS 4D 5M, 2015 GSW SE 6M
Thanks. The glow plug light is flashing.
Right, but unless you believe the primary problem is the glow plugs or gp system, this gp thread isn't the best spot to get views and replies. Many times starting a new post isn't necessary because the question has been asked and answered. In your situation, starting a new thread in the appropriate spot will be more productive, and not derail a glow plug discussion. If you have VCDS, you can also add the actual code associated with the warning lights (flashing gp light is a general engine immediate warning).
 

rustman1984

New member
Joined
Jan 10, 2024
Location
San Antonio
TDI
2003 Jetta TDI ALH manual 5-speed
ything you can check. If you haven't found any problems yet, then you likely have a bad harness. There is nothing wrong with the integrity of the harness, its just a bit of corrosion or oxidation in the terminals that is causing poor contact, resulting in the high resistance that is making the ECM think its a bad
Well, its been over a year since my original glow plugs 101 thread. I just got the code back so I had to do the testing procedure again and thought I would redo my How to with new pics & info. So here goes.

The glow plug codes are a popular code to be thrown on our cars. Several versions of the DTC are

16764 / P0380 code (Glow plug/heater circuit electrical fault (Q6))
17055 / P0671 code (Cylinder 1 Glow plug circuit (Q10))
17056 / P0672 code (Cylinder 2 Glow plug circuit (Q11))
17057 / P0673 code (Cylinder 3 Glow plug circuit (Q12))
17058 / P0674 code (Cylinder 4 Glow plug circuit (Q13))

The first code is for 2001 and older cars. They have a 2 wire glow plug harness, so just throw a generic code for the glow plugs.

The other codes are just examples of codes you might find on a 2002 or newer car. They have a 4 wire harness, so the ECM can specify the problem plug.But the trouble shooting procedure is the same for all codes.

NOTE: I have tested a few 2002's recently for glow plug problems. I don't know if this is the case for all of them, but the ones I tested have the cylinder order backwards. For example, if the code specifies a bad plug at cylinder # 1, then cyl # 4 may be bad. The last car I tested had 2 bad plugs according to the codes. Cylinders 1 & 2 were indicated as being bad, but actually it was plugs 3 & 4 that were bad. So don't always trust the codes. Check for yourself with the multimeter before replacing any plugs.

Now why was a code thrown? The ECM measures the resistance accross the harness wires. If it finds an imbalance in the resistance between the wires, It thinks there is a bad plug since a bad plug will have high or infinate resistance. But the big problem is that there are other factors that can cause the resistance deviation other than a bad plug. A bad relay, corroded or oxidised harness, a bad ground or a blown fuse can all cause the code to appear. So I will go step by step through each of the possibilities so you can test and rule out or confirm the actual cause.

The first step I suggest is to check the plugs themselves. Afterall, they are the intended culprit that should light the MIL. What you need to do is remove the engine cover and locate the glow plug harness. You will need a 10mm socket & a digital multimeter for this step.

Here is the harness:


To remove it, just hook your finger under each terminal and pull. they snap on & off like spark plug boots. Once it is off, just rotate it out of the way so you can expose your plugs:


Then get your digital multimeter (available at any auto parts store or Wal-mart for less than $20) and set it to the lowest ohm setting possible (usually 200). Then place one probe to a good, clean ground and the other probe to the tip of the glow plug:


Do this for each plug and take note of the resistance value of each plug. They should all be between 0.5 and 1.5 ohms. It doesn't matter what the readings are, as long as they are close to eachother (within 0.2 - 0.3 ohms). If you have a plug that reads high or infinate, then you have found the bad plug and it needs to be replaced.

EDIT: I have measured sets of plugs that have had a 0.1 ohm difference with no code being thrown. But one member who contacted me had a code thrown with a resistance of 0.6 ohms on 3 plugs and 1.1 ohms on one plug. This was enough of a variation to throw the code. So even though we do not know the exact tollerance difference, its somewhere between 0.2 and 0.5 ohms.

Replacing plugs is very easy. Its like changing a spark plug. You need a 10mm deep socket and an extension. Many reccomend a 1/4" drive socket to allow the extension to get around the injector lines, but I have done them with 3/8" drive sockets too. Or, if you have a wobble head extension, even better. Just spin out the old one & spin in the new one:


I strongly suggest putting in the new ones by hand. You want to make sure it spins freely before using the ratchet on it to make sure it is not cross threaded. start it by hand and spin it in at least 3 turns to make sure it doesn't bind and then finish withthe ratchet. Tighten to 11 ft/lb. If you don't have a tourque wrench, no problem. Just make them snug, but not tight. Grip the ratchet with the head in your palm so you can't use the leverage of the handle and tighten it that way. That should be close to the correct tourque without overtighteneing.

FYI, There are also 3 coolant glow plugs on the manual transmission cars. These are 12mm plugs as opposed to the 10mm ones used for starting the engine. The purpose of the 3 coolant glow plugs is to assist in heating the coolant. They are not necessary for starting and are NOT MONITORED by the ECM. So there is no need to check them to solve any codes. But it can't hurt to check them anyway while you are under the hood if you want to make sure they are OK. I have tested many of these coolant glow plugs and have never found a bad one. I suspect that they last much longer since they are not exposed to the same harsh environment of the combustion chamber like the engine glow plugs.

If your plugs all checked out OK, then you have another problem. It is likely your harness, but since there is no way to test the harness, we have to move on to other steps to rule them out. If all other tests check out, then that only leaves the harness, so we will address that last.

The next thing to check is the relay. To check the relay, you need to see if your harness is getting battery voltage. In order to do this, you need to unplug the coolant temp sensor. This will trick the ECM into thinking its really cold out and activate the glow plugs for about 20 seconds. This will give you enough time to turn on the ignition, run to the front of the car and perform the test. Here is the coolant temp sensor:


It is located in the housing below and behind the 3 coolant glow plugs(manual transmission cars). It is difficult to see, so you may have to feel for it. It unplugs in a similar way the MAF unplugs, just a smaller plug. You need to push on the tab with your thumb to release the clip while wiggling the plug off . Practice on the MAF if you want to get a hang of it.

Once it is unplugged, set up your multimeter to measure voltage. You need to set it to 20V on the DC side of the meter. Turn on the ignition (do not start). Then find a good ground and place the other probe into the end of the harness terminal like this:


You should get around 12V at each finger of the harness. The glow time is only about 20 seconds, so you may have to repeat the procedure to get all four if your not fast enough. Once the glow time is up, the voltage will drop, but it will still show a small votage at the harness terminals:


I'm not sure why there is still voltage, but it is normal. If you do not get 12V at any time, then your relay is not completing the circuit and needs to be replaced. The relay is an expensive part (around $125), but fortunately, it is not often the cause of the code. I have only tested one system out of dozens that I have tested that ended being a relay problem.

So if the relay checked out ok, then you need to check the fuse under the battery. This was why my MIL came on by the way. Until recently, I was driving around without a battery cover and the fuses on top of the battery were exposed to the elements. They became oxidised & corroded. Two of the big fuses were not conducting current between the two posts. I tested them by placing the multimeter on each post to see if there was voltage to both sides of each link:


In my case, I got no reading on the far side of 2 of the links. The glow plug fuse is the second from the right. I removed the fuses, cleaned them with sandpaper and reinstalled them. I also used some Deoxit D5 to help enhance the contact & remove the oxidation. If your fuses are clean looking, I wouldn't worry about it, but it can't hurt to check. Of course, if one is blown, then replace it.

So now you have checked everything you can check. If you haven't found any problems yet, then you likely have a bad harness. There is nothing wrong with the integrity of the harness, its just a bit of corrosion or oxidation in the terminals that is causing poor contact, resulting in the high resistance that is making the ECM think its a bad plug. Most of the time, replacement of the harness is not necessary. It can be treated with the product I mentioned earlier, Deoxit D5. I have used it on many cars with great success. Here is a link to a discussion on this product.

In some cases, the harness may be too badly damaged to be treated. This would only be the case if it was left untreated for any length of time. I had a realy bad harness when I bough my car, so I replaced mine. Here is what I found when I opened it up:


As you can see, the oxidation on the two center liners was quite evident. They are white in color instead of shiny like the outer ones. The poor contact created by the oxidation caused arcing when the voltage was trying to complete the circuit. The arcing caused those black scorch marks. This obviously caused a poor connection which resulted in the high resistance, causing the code to be thrown. This harness caused my CEL to come on many times. Once cleared, it would always return. Sometimes it would stay off for 100 km of driving and sometimes it would come back within minutes. After I changed the harness last December, I have not had a glow plug fault now for almost 18 months and 60k km. But as I said earlier, don't consider replacing it until you try to treat it. No point cutting out the original one and splicing in a new one if you don't have to. Treating it is cheaper, easier & less invasive that replacing it. But if you do feel you need to replace it, here is how I did mine:

First, cut the old harness off with wire cutters. To make it easier to splice in the new one, cut the wires as close to the harness as possible so you are left with longer pieces to work with. In my case, since I was redoing my connection just for the purpose of taking pictures, I cut mine at the old joint:


The new harness (two wire version) comes with spade connectors on the endes of the leads. Just cut them off since there is nothing to connect them to anyway. Then strip about 1/4 inch off each lead:


Then crimp on some butt connectors to one set of wires. I use uninsulated ones since I can get a better crimp with them. They will also look nicer (less bulky) under the heat shrink tubing. I got them from Radio Shack:


Then I slid on a couple of pieces of heat shrink tubing and crimped the other two wires to complete the splice:


I then used a heat gun to shrink the tubing to seal the joint:


I used heat shring tubing with a glue lining. When it shrinks, the heat melts the glue and it makes a water tight seal around the joint. I bought it at a tractor trailer repair shop. They use it for wiring trailers since the wires are exposed to the elemnts (rain, snow & salt). They are very good. You will also find it easier to do the crimping if you remove the vacuum ball and set it off to the side. There is more room to get the crimping pliers in there.

If you have a 2002 or newer car, then your harness has a long set of wires and can be plugged into the main harness. But it is somewhere in the main loom and removal of the air filter box and possibly the battery too to route the wires the same as the original harness. It can be quite a pain in the butt to replace. A good reason to try to treat it rather than replace it.

This will give you an idea of how to test & diagnose your glow plug fault and save yourself an expensive visit to the dealer. I have read of quotes between $500-700 to fix these codes. The plugs are available from www.tdiparts.com for $80 / set shipped and Deoxit D5 is available at lots of places online. Just do a google search for it. BTW, you need the 10mm plugs for the engine and the coolant plugs are 12mm.

Good luck and hope you can get rid of that pesky code for good with this procedure.

EDIT: TDI Allen did a writeup on how to change the 4 wire glow plug harness if you decide to change yours. You will see that it is quite involved, so that is why I suggest treating you harness before considering replacing it. Here is the link
Thanks for the write up. Could you clarify one bit for me? It may be an oversight on my part for not understanding, apologies in advance if that is that case. You said that the harness should be tested last. After testing the glow plugs, you said to test the relay next by measuring the voltage at each harness glow plug end. My question is this, isn't this method really testing both the harness and the relay at the same time? If there's no voltage, then either could be bad? I suppose though, that the odds of all the harness glow plug ends being bad at the same time is pretty low. This means that as long as any of the harness glow plug ends are giving voltage, that the relay is good and the harness is the issue? To clarify further, any voltage whatsoever at the harness ends means that the relay is working correct? It's either allowing current through or it's not? Thanks again!
 

Aj112012

New member
Joined
Jan 3, 2024
Location
Texas
TDI
Passat
Ok I have glow plug light blinking, this post was helpful. I have replace all glow plugs, and a sensor connected to them. The code was for cylinder 1 firing and now it’s gone after the replacement. However the CEL is for P0113 AIT sensor.

I took the MAF out and cleaned it and replaced it. Reset the code and then it came back on test ride.

Also replace fuel filter.
 

BobnOH

not-a-mechanic
Joined
May 29, 2004
Location
central Ohio
TDI
New Beetle 2003 manual
The blinking glow plug light is not the the plugs. It's simply to get your attention.
CEL link to Ross Tech
Sounds easy. As these cars age, we see a lot of simple wire/connect issues. Simple problems, not easy to find.
 
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