BMW Check Engine or Service Engine Soon Light – Thermostat Fault Code
This answer is applicable for many BMWs in addition to the one listed below.
car year: 2004
car model: 325Xi
Last month, I noticed the service engine light came on when I was driving. It did not go away the next few times I drove it. So, I ordered a diagnostic kit (fault code reader/reset tool) from your catalog. While waiting on the kit to arrive, I took my car on a short trip and I noticed the light had gone off. When the part arrived, I did the check and got the fault code for thermostat. Does the light going off indicate that It was only an intermittent problem? I’ve been told that thermostats sometimes stick.
The thermostats in late model BMWs (most models 1999 and later) are electrically controlled. When the signals through the thermostat circuit are not what the engine management system expects to see, it will generate a fault code (even if the temp gauge seems to read as it should). Often the fault light will go out, but the code is still there. You likely do need to replace your thermostat (thermostat failures are very common). If you want to check this, to be sure, check the current fault codes (record them) then clear the codes and see if the same fault codes return. If they do, then you need to address whatever issues apply to the faults (such as the thermostat).