Wednesday, December 23, 2009

Just out of curiosity

Does anyone think a bad temp sending unit would light a SERVICE ENGINE SOON light?

It just came on after work tonight and I saw the water temp gauge was only reading half of what it usually does.
I'm wondering if that three wire sending unit is causing it.


  1. On MOST* vehicles, the temp gauge sender and the coolant temp sensor are separate units.

    The CTS tells the engine computer how warm the engine is so it can adjust the fuel parameters for best emissions performance.

    If the ECM doesn't see coolant temps in a certain range after a specified time interval it sets a diagnostic code and turns on the lamp.

    Presuming you are in an area that needs it, how was your heater / defroster working? If the thermostat is leaking or stuck open the engine will be slow to come up to temp and will cause the scenario I described above.

    *I don't want to hear about the 2010 Whizbang that has one sensor that reads engine temp, oil pressure, relative humidity and how much change you have in your right pants pocket. I'm making a GENERALIZED STATEMENT in answer to a question with limited detail!

  2. I had a scan run on it at Auto Zone for free- it's something in the O2 sensor circuit.