P0128 thermostat temp sensor problem

[StNickM]

Ok, car is 2002 M3 obviously

So I've been fighting this check engine light for a minute now, and am now to the point of it driving my crazy. I keep getting P0128 code, which usually is associated with the thermostat sticking. So I went and got a new thermostat from the dealer and installed it along with some fresh BMW coolant. The old thermostat didn't look bad, no corrosion or anything. The new one looked and acted the same. (I'm still not sure how these type of thermostats work as it looks like its basically strait through flow.) So put that in cleared light and sure enough a day later the code is back. Occasionally the code will go away on its own but recently Ive been clearing it daily and its coming back daily so whatever the problem is (also its cold now so that wont help the warm up time) is worse. Ive used a obd2 tool to read coolant temp and from what I see the car is running around 158-160F just driving around and highway. I did the dash test thing and it was reading 67-68 degrees (started at 11 or so on a 50degree morning) on the dash in mode 19 test 7.0.

So do you think the thermostat is bad even though its brand new, or is it one of the temp sensors. Further more WHICH of the damn sensors feeds this info. I thought the one on the rad hose fed the dash and the other the ECU but I'm not entirely sure.

Please help I need to get this light to go away and get the car inspected.

[dworthy]

Did you also get a code for coolant temp plausible? This is a very common code on the S-54 motors, and it will be the coolant temp sensor that is right behind the thermostat. The fact that you are getting a P0128 is questionable anyway as that actual code as far as I know was never written into the S-54 diagnostics. That code is for the electrical controlled thermostats that are found on the M-54 and similar motors.

My advice will be to find a proper diagnostic tool, as the pocket scanner you are using will not cut it on this car, and the "P" codes are just a generic way to read them.

[jclausen]

Replace temp sensor