My check engine light came on about a month ago, and when I took it in to the dealer, the code said the coolant temperature was below normal.They reset it, but about a week later it came back on. I went back and they told me it was probably the thermostat and it would cost $550 to replace it. I bought the thermostat from them and spent about 4 hours with a shop manual in hand attempting to install it, but I got to the part where I had to disconnect the fuel line with a tool I did not have, so I had to just put everything back, without fixing it. I got the tool, but I'm not sure that’s the problem anymore and don’t want to spend 6 hours replacing something that doesn’t need to be replaced. My dad said it couldn’t be the thermostat because the car starts to heat up in about 3 minutes of driving, and is completely warm after about 5 minutes. Does anyone have any ideas? It’s an 03 CTS with 120,000 miles on it. Thanks!

 

It would help if you posted code, I will guess P0128. PCM expects coolant temp to rise to operating temp in a certain length of time, if it doesn't code sets.Also installing lower than stock temp thermostat can turn turn light on.

 

Thanks for your help slowroll, the code is P0128. I got the car with 115,00 miles on it, so the thermostat might have been replaced with one that is lower than stock... I dont know. I wonder though if maybe it might just be a bad sensor or somthing since the car warms up pretty quick.

 

ECT sensor has its own codes. A skewed sensor is always possible, however you will be guessing with your money.