Glow plug issue

2001 TDI Jetta and the glow plugs do not come on until after the engine has started. We have followed the troubleshooting threads for glow plugs and everything checks as OK to the last step which is to replace the temp sensor. We have tried 6 different temp senors and still the same issue. The glow plugs work when the temp sensor is unplugged.


Where to go from here? Which wires on the temp sensor are for the glow plugs?
Thank you

The fact that you're getting afterglow *and* a full glow cycle with the CTS unplugged suggests that the system is largely working as designed.

The plugs themselves don't pre-glow much until around the freezing mark. Unfortunately not too familiar with Iowa weather... is it around below freezing where you are?

If so... the glow plug control module is probably your next suspect... it's what decides how long run the plugs based on temperature.

Vince Waldon said:

If so... the glow plug control module is probably your next suspect... it's what decides how long run the plugs based on temperature.

Click to expand...

It's an 01 I don't think it has an indipendant GP controller.

The component I'm referring to is P/N 038 911 253, position 10 on the relay panel, usually has a "180" label on the top.

It's a relay plus some electronics that reads the CTS and decides on pre-glow time.

Vince Waldon said:

The component I'm referring to is P/N 038 911 253, position 10 on the relay panel, usually has a "180" label on the top.

It's a relay plus some electronics that reads the CTS and decides on pre-glow time.

Click to expand...

Yeah you're right, was looking over the schematic on it Try another one I suppose. I wonder if it'll log a code in VCDS if it goes bad.

I've read the GP system monitoring is pretty good and usually will trip the appropriate code, but it also sounds like nothings left but the relay. Do check the associated wiring/connectors.

No worries.

The ECU has a pretty good handle on overall glow plug system operation and the specific health of each glowplug, but there are some subtle things it can't detect, such as a CTS sensor that's lying about the actual temperature or if the module itself is making a bad decision.

The OP's issue seems to be on the subtle side, since it sounds like he's getting proper afterglow power to the glowplugs *and* a new CTS / proper response when the CTS is disconnected suggests the sensor side is fine.

Expensive part to shotgun, unfortunately... maybe a local junkyard has 'em?

Does the glow lamp come on when you turn the key on and go out after an appropriate time period on a cold start?

Does the engine start up OK cold?

Does the ECU see the coolant temp on a cold start as it actually is?

There is a preglow period somewhere in the ECU data (I forget where, rarely need to look), I'd be curious to see what it says.

49ish F is the threshold for preglow, but there are other factors like altitude pressure figured in to the preglow map , not sure how much difference it actually makes.

The glow control unit inside the car can fail and set a specific DTC, that is NOT an actual glow plug DTC. I have seen that a couple times, the MIL will be on though, as it would be with any specific plug circuit failure. These cars are actually VERY good at monitoring the glow system.

Just to be clear, do you have a glow plug code of some kind that is causing your engine light to come on?



Check the wiring to the temp sensor. Be especially observant of where the wires go into the CTS plug itself. I've found several VW wiring problems at the plugs themselves. If that checks out, follow the two wires as far as possible and check for chafing/breakage.


You have an 01 so you have a two wire glow plug system, and the relay is located under the dash above your feet with a 180 printed on it. If you don't find any wire compromise you could try changing that out and see if it fixes the issue.

10.8 C likely will not do any preglow, it is right on the threshold. Look to see what the preglow period is supposed to be, I bet it is at 0.00 sec. You have something else going on, and the extra aid of preglow helps get it going. Low compression, poor fuel injection, slow cranking, bad fuel, etc.

Unless the CTS value is off "just enough" but usually they either work or they do not. Even with no functional preglow at all, a healthy ALH will start right up at 45 F.