My 330 XI 2001 is giving me an error code P0128. The check engine light goes on and off after a few days. The engine temparature is dead center after only a few minutes of driving.

Is really the thermostat the problem even though the temperature is OK? If so, how much BMW coolant do I need if I replace the thermostat?

Thanks,

You double posted this and forgot what could be an important piece of information in this one.

and it was like that for about two weeks until I decided to turn on the AC, then the light came up again. Same error code P0128. Then a few days later it went off again until I turned the AC on.

From what I've read, the temp gauge will sit in the middle through a really wide range of temperature in these cars. I know my car gets to the middle really quick and stays planted there whether it is 50 degrees out or 90 degrees but my oil temp will fluctuate with ambient temp changes so I'd tend to believe it.


Things it could be in order they should be checked.
Low engine coolant level (you already fixed this)
Leaking or stuck open thermostat
Faulty cooling fan (running too much)
Faulty coolant temperature (ECT) sensor
Faulty intake air temperature (IAT) sensor

I believe the 330 XI has an aux cooling fan that comes on with the AC. Prolly why you are getting the code only when you turn the AC on. I'd go with the stuck open thermostat and the aux cooling fan brings your temp down below tolerance for the PCM and you get a code.

Replace the thermostat. Sorry, no idea on the coolant requirement for replacing the thermostat.

Thank you for you help. I couldn't find the other post. That is why I posted again.

How often do the ITA and ECT sensors need to be replaced?

When they fail, no particular expected lifetime that I have seen. They are your least likely suspects though at this point. That is assuming you have gotten no further than asking questions.

I'd really change out the thermostat and bleed the coolant system and see where you are if it were me. They are $90 at my local Autozone, maybe cheaper somewhere else.

If you want to test the one you have, remove it and put it in a pot of water with a thermometer. It should stay closed until about 200-206 degrees.