The car is a 2003 VW Golf GTI 1.8T. Recently the MIL light came on and it was diagnosed with a defective secondary air pump and thermostat. I had them replace the secondary air pump only. The thermostat was not replaced because the engine is not overheating and the temperature gauge reads normally at 90 degrees Celsius.
The question is would a malfunction thermostat causes the MIL light to come on?
What was the code that it recorded?
Damn near anything will cause the light to come on. Knowing the light is on, though, tells you absolutely nothing except that an error of some kind has been recorded. Without knowing what that error is, you don't know anything.
A malfunctioning thermostat could cause the engine to be running too cold, and that could cause problems that could set codes. If the heat doesn't seem to be working very well, a jammed thermostat could well be the cause. Then again, it could be something totally unrelated. Without knowing the codes you don't know.
--scott
It can, the ECM times how long it takes for the engine to reach a certain temperature. If it takes longer than the timer allows then the ECM will command the check engine light to come on.
What you need to keep in mind is that an OBD-II diagnostic code indicates a problem in a CIRCUIT, not necessarily a sensor. It might very well be a bad thermostat, but it might also be a bad temperature sensor, a broken or loose wire, a bad ground, and so on. The code will usually point in the general direction of the problem, but it will not tell you everything you need to completely diagnose and fix the problem.
Chris
On this topic, I've got a '93 Grand Am 3.3L that flashes a code - can't remember the number now - indicating "quad 4 driver." The symptoms are the engine dying or almost dying. Seems to go away for a while when the engine cools down. Daughter's car and it's never happened to me. This is OBD-I. Really haven't found anything on this that can give me a starting place to solve it. My thought is to take it to a dealer where they can put a scanning tool to it. Is this realistic? Don't know diagnostic tools except paper clips and code readers myself.
--Vic
I have read through the replys that has been posted to this and i would have to agree with everything they have said but i do want to touch on the one where it mentioned that the cars computer is programed to know how long the engine should take to warm up based on several parameters. If your car engine doesn't warm up in this amount of time then it cannot go in to "closed loop" operation and will still sense a fault, and remain in "open loop" mode. One thing to keep in mind scan tools are not a fix all, they are just another tool in your tool box and you'll still have to do some old fashioned trouble shooting and research to figure out how the operating system works and why it is in a state of imbalance
I don't know. I was not told what the code and did not thought to ask at the time.
I believe the engine is not running too cold. Although I have not precisely measured the time it takes for the temperature gauge to indicate 90 degrees Celsius I believe it does it in 5 minutes or less.
Yes. P0128 would probably be the code. You have to know how the computer rationalizes the engine temp and warmup time to diagnose. It's different from make to make. Even model to model. Sometimes if the ambient air sensor or air intake sensor is bad, this code will set. Bad head gasket, low coolant, intermittent cts. A lot of cars have idiot temp gages that don't move with fluctuating water temp.
You may want to check the temp sensor before replacing the thermostat in case it appears to run at normal temps.
Well, don't worry about it, then. If it happens again, find out what the code is and then you can go from there. Without the code, anything anyone can suggest will be a total shot in the dark.
--scott
The temperature sensor was replaced in April 2006 or approximately
52000 kilometers ago.
So that sensor is about the same age as the one you replaced back then.
It is probably much easier to diagnose and replace the sensor than the thermostat.
I don't know how the golf sensors are, but I've had problems with temp sensors in several of my audis, and initially thought it was a thermostat problem.
Normally they have one sensor for the engine management system and one for the gauge, but they are often mounted in the same housing. The temp gauge may indicate normal temp, but the ECU sees the wrong temp, or vice versa.
The fault code is P2181.
What effect does a "temperature below control range" have on the performance of the engine?
It'll keep running open loop, you won't pass emissions, bore wear may increase due to overfueling
nate
That does indeed sound like it's running too cold. You sure the gauge is reading correctly?
The ECU won't be able to go into the normal running mode, so the engine will likely not be running with the right mixture. On top of that, you will be getting lousy gas mileage and putting additional wear on your engine. Definitely a thing you want to fix immediately. Oh yes, and your heater won't work worth a damn.
Check the thermostat.
--scott
Your fault code looks an electrical issue.
The needle is pointing straight up.
There's plenty of heat coming out of the heaters even when the ambient temperature is -10 degrees Celsius.
Where is the temperature sensor located?
Easier way to determine where the problem is. Grab a mid-line code scanner that has data display. Plug it in and see what temperature the computer is reading from the sensor. If it's reading way off what the gauge reads you can then look for the bad wiring/sensor involved.
On older VW:s (mid 90:s) it is mounted in the plastic fitting mounted on the cylider head to which the upper radiator hose is connected.
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