LordKarnage17

Its just starting to get cold around here and I've noticed that every now and then on a cold start the engine wont hold a high rpm like it should and misfires, sometimes getting bad enough to stall the engine.
Till now I assumed it was the coils or spark plugs or something obvious like that, but I recently used my obd reader to check the codes (I've got codes for the abs module crapping out so I cant tell from the light when a new code comes on) and saw a new code for coolant temp sensor reading high. The gauge on the dash works just as it should, so I never suspected that to be the issue, but if it read high the ecu woudnt richen the AFRs which I would assume could cause my symptoms during cold starts. It runs completely fine once warm. The cooling fans also run at full speed during these cold starts so the sensor reading high would make sense.

Assuming the sensor is the issue, is there more than 1 and would it be able to be replaced without draining all the fluid? its a 2003 b6 a4 5spd quattro

Also are the sensors interchangeable on 1.8ts? because theres a 1.8t passat in a nearby junkyard that I could get the sensor from.

lyleswk

There are 2 sensors on the US cars and I have no reason to believe yours is different. There is one located at the back of the engine near the firewall and one in the hose that connects to the bottom of the radiator. They are different sensors, so keep that in mind. If your gage works properly, it is probably the lower sensor which means you have to drain the coolant. I also have no idea if the Passat sensor is the same. Not sure that would be the cause of your misfire, but should fix your fan issue.

LordKarnage17

So what you're saying is that the one close to the firewall is the one that controls the gauge, and the one by the rad is the one that controls the fans? I took a photo of the green ECT I got from the passat and went to the dealership to ask if I could see the replacement part for my ECT and they looked identical, so I'd imagine they would be interchangeable unless the resistances vary or something weird like that...

lyleswk

Both are used by the ECU to control various things directly, such as the thermostat, timing, cold start, etc. It is a fairly complicated algorithm. If one isn't working, it will turn the fans on all the time to trying to keep things from overheating. Finally, the top one directly signals the gage.

One final note, the top one is also known as the "green" sensor. It is a four wire sensor. The lower one is known as the "black" sensor. Believe it is a two sensor wire. Looks like the dealer has confirmed they are interchangeable. I will repeat again that I think your issue is the lower sensor since your gage is okay.