Coolant Temp Sensor Replacement

lincolndj · Jul 9, 2014

Hi guys, first time poster on here, as I’ve just bought a 2003 B5.5 Passat 1.9,

On driving it home from the garage I bought it from, I noticed the engine not warming up (never went above 55°c), so assumed either stat or CTS. There is no dash warning lights and no fault codes showing on VCDS.
Next morning, with engine cold I compared the ambient and coolant temps with VCDS (should be close with a cold engine), ambient was about 18°c and coolant was around -13°c which accounted for the approx. 30-35°c short-fall to 90°c on the dash.
Also, the temp didn’t go up after driving hard then being stationary for a while (which if it was the stat the temp should at least rise when stationary and engine running – unless a fan is stuck on too?!)

Took car back to where I bought it from and they’ve replaced the CTS. But the car didn’t really get any warmer when driving back home so now I’m not sure what else to check. (Would just take back to garage but it’s a 2 hour+ round trip, if I can diagnose myself first it may save more trips)
Are there 2 different temperature sensors? Eg one that turns the fan on/off and another for the ECU/gauge? What else should I check in VCDS? The garage replaced a sensor that looked near the bottom of the rad, below alternator.

Also, any tips on how exactly the control logic works? Eg from a cold start, with A/C switched off, should any fans be running from the off?

Cheers, Adam.

B100 · Dec 15, 2014

Can someone comment on what effect a dashboard temp gauge reading low (between the three tight bars on left and 190) will have on engine performance? My 2003 Golf would not get up to temperature on a long drive yesterday, but also felt like it had lost power, just did not want to accelerate up hills in 5th gear that would normally quite happily speed up on.

I'll log data with VCDS this week, didn't have it with me on the drive.

No CEL, CTS replaced maybe six years ago, we've been having wet weather and I've been running with cabin heater on... nothing else to explain what might have changed, apart from swapping the H7 bulbs...

B100 · Jan 8, 2015

It was a bad temp sensor... I just replaced, and dash needle swings at normal rate up to 190. Engine feels happier, too.

Now, where did all the G12 go?

TonganDiesel · Oct 4, 2015

tkindler said:
Remove the engine cover and on the driver's end of the engine, look for a part with three plugs sprouting from the top as in this picture:

http://pics.tdiclub.com/showphoto.php?photo=30802&cat=500&ppuser=16933

On the back side toward the bottom of this part is the coolant temperature sensor. It has an electrical connection with a wire coming from the back side of it. It is held in place by a little plastic clip which can be pulled off toward the bottom of the sensor. Be gentle, the clip can break, and if you drop it, have fun looking for it. You will need it to secure the new sensor. Some folks suggest having a replacement available, just in case. The steps I followed when I replaced mine are as follows:

0. It is best to do this while the engine is cold.
1. Open the top of the coolant overflow container to relieve pressure.
1a. You will probably want to remove the large hose running from the airbox to engine. This will make access to the sensor easier.
2. Remove the electrical connection from the old sensor
3. Connect electrical connection to the new sensor, so if I drop it, it doesn't get lost.
4. Remove the retaining clip holding the sensor in place
5. Be sure you have the new rubber O-ring on the new sensor
5a.Remove the old sensor, being prepared for coolant to come out.
5b.Pull and twist the sensor, it should come out fairly easily.
5c.Check to be sure the rubber O-Ring came out with the old sensor
6. Quickly insert the new sensor with the new O-ring to prevent coolant from leaking
7. Re-attach the plastic clip to secure the sensor.
8. Check coolant level and, if necessary, top up with the recommended mixture of G12 coolant and distilled water. I used 50/50, although I don't think that is the exact recommendation. If you don't have G12 coolant use distilled water only.
9. Put top back on coolant overflow container.
10 Re-attach any other parts you removed.

Note: Yours is a PD engine, mine is an ALH, but I think all this applies to your engine. Someone else chime in if I am mistaken.

What are those three plugs spring from the top of that coolant housing you mentioned? I can't find a reference to them

hskrdu · Oct 5, 2015

TonganDiesel said:
What are those three plugs spring from the top of that coolant housing you mentioned? I can't find a reference to them

Aren't they coolant glow plugs? I can't see the pic.

Ed ke6bnl · May 17, 2016

NeXTstep guy said:
Could someone give me some really, really, really, detailed instructions about how to disconnect the connector? I tried (incorrectly/unsuccessfully) to remove this type of connector on my MAF. It befuddled me. What's the secret?
RWW

this video seems to give a good demo, I believe I will be doing it next week
https://www.youtube.com/watch?v=j7DozEry0hg

andrewfoster · Mar 1, 2017

A postscript to a rather old thread, just in case anyone still needs it. As the car ages, remember that there is corrosion everywhere, although I've been careful to change the coolant as prescribed. The coolant sensor (BEW engine) was very difficult to remove, and the O-ring was stuck firmly, needing to be pried out. I had to make a small scraper to clean out the housing before I could get the new sensor and O-ring in, and it was a struggle. It took over an hour to do the whole job.

B100 · Mar 1, 2017

B100 said:
It was a bad temp sensor... I just replaced, and dash needle swings at normal rate up to 190. Engine feels happier, too.

Now, where did all the G12 go?

And here we go again... temp needle not swinging to fully erect, car feeling slightly sluggish... could it be the two-year-old sensor has gone, already?

netjustin · Mar 4, 2017

andrewfoster said:
A postscript to a rather old thread, just in case anyone still needs it. As the car ages, remember that there is corrosion everywhere, although I've been careful to change the coolant as prescribed. The coolant sensor (BEW engine) was very difficult to remove, and the O-ring was stuck firmly, needing to be pried out. I had to make a small scraper to clean out the housing before I could get the new sensor and O-ring in, and it was a struggle. It took over an hour to do the whole job.

Recalling my recent (also BEW) CTS replacement, once the electrical connector and c-clip were pulled I was able to rotate and shimmy the sensor in place.

Not sure if the shimmying freed up the o-ring, but FWIW the o-ring came out on the sensor.

Coolant Temp Sensor Replacement

lincolndj

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B100

Veteran Member

B100

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TonganDiesel

Well-known member

hskrdu

Top Post Dawg

Ed ke6bnl

Veteran Member

andrewfoster

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B100

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netjustin

Active member