Check engine light was on and now it's gone off?

(2011-11-21 15:20:23)

1. Lulu Marie Says: My check engine light came on a little over a week ago and when I took it in for a diagnostic they told me my car was running cooler than normal so I was planning on replacing the thermostat tomorrow. Today when I got in my car the check engine light was off. Do I still need to replace the thermostat or should I wait?

2. Michael Says: All Late model cars have a minimum coolant temperature. For OBD-II vehicles, if the engine runs too cool, the ECM will set a code for "coolant temp low", P0128 I believe. Late models usually run about 205 degrees normally. I have a 2004 Trailblazer and a 2005 Envoy and both set a code for this. I had to replace the thermostats on BOTH vehicles at 140K miles. Now they run at 205 and they are just fine. The engine temp may only drop 10-15 degrees, but that is enough to set a code. Replaced the Thermostat and the code went away. This code is more noticeable in cool or cold weather.

3. METROPOLIS1 Says: Many people mistakenly think when a check engine light comes on there is some horrible problem and it doesn't turn off..... In fact if there is a sensor telling the computer there is a problem then the light comes on - AND if the sensor detects no problem because the parameters have changed then it will turn off......

You may have a part, maybe the thermostat that is occasionally operating out of its design parameters so this is why the light turned on, then cleared itself.....

Anyways - a thermostat is very important to your engines efficiency.... Have this changed and while your at it have the cooling system flushed and refilled with new coolant.....