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Check engine light

I have a 2003 VW Jetta TDI and the check engine light comes on periodically, and stays solidly lit for a day or two. The it goes off and stays off for a day or two. I took it to my local dealer who tested the computer and said that it was the thermostat and a related sensor, but that story doesn’t seem to add up - no temp problems and the light comes on when its hot or cold.

What was the code? When the CEL is lit, the computer sets a code, like P0132. Even if the light goes out, the code should remain in memory. This code is very important to get a proper diagnosis. It may be on the receipt or work order from the dealer.

There could be a hundred codes related to the thermostat and temp sensor. Each code has a more specific definition that can narrow down the problem.

If the receipt does not indicate which code is causing the the CEL to light, get the code from an Autozone or Advanced Auto Parts store. They will come out to your car and get the code for you. You need the number of the code, ask for that specifically. Then get back to this page and report that number back to BustedKnuckles.

Thanks, I just now retrieved the receipt and it does not state the specific code. The receipt says: “Found fault present for cooling system performance. Found thermostat opens at 64 degrees causing engine to run cold. Recommend replacing thermostat and eng coolant temp sender. Continuing to operate vehicle with this concern could cause future issues with the exhaust gas recirc system possibly plugging system with carbon prematurely.” I don’t see how they could say so definitively that the thermostat opens at 64 degrees and how that would cause the light to come on, when the engine is hot or cold.

 [i] The story does add up.  The light does not sync with the problem.[/i]

 It could take a certain combination of things to trigger the light.  

 The important thing for you right now is to tell us what the code was {format = P1234}  Codes No code says "it was the thermostat and a related sensor"  That was the mechanics interpretation.  Give us the real code and we may be able to give better advice.  Many auto part stores will read the code for free.

Thanks, will do

The computer system, known as the ECM (electronic control module), in this car is very sophisticated. The ECM set a code and the CEL popped up because the ECM had trouble getting the engine up to temperature. I’ll bet that the mechanic ran some tests after getting the code, and determined the thermostat is opening WAYYYY to early. There are scanners that can read the sensors in real time, just like the ECM is doing, and show the mechanic what the computer sees.

With the thermostat opening too early, the car doesn’t warm-up properly. The proper engine temperature is important for the ECM to keep the fuel mixture at it’s most optimal. The ECM ‘sees’ the cool engine and enriches the fuel mixture to try and get it to proper temperature. This enriched fuel mixture means your gas mileage is bad, and that there is a lot of carbon in the exhaust that can plug up the EGR and catalytic converter. This carbon plugging will cause more damage down the road with increased emissions, reduced performance, and worsening gas mileage.

If you have not had the cooling system flushed and filled, this would be the perfect time to do it. Get the cooling system serviced with a clear-water flush, new clean coolant added, along with a new thermostat and new radiator cap. And, have them check the condition of the radiator hoses and heater hoses. After 7 years, some may need attention.

OK so I finally got the code, the receipt reads: “Code P0674 which is #4 glow plug circuit - Q13 - electrical fault sporadic.” Any wisdom out there as to what this means?