I have an 06 Cobalt SS with a 2.4l. My coolant temp sensor will go out shortly after I start driving and show --- . I've changed my sensor, wired in a new harness for it, and my thermostat. Traces my entire wiring harness and didn't find any shorts or bare spots. What else could it be? I can pull my battery cable when my car is off to reset my computer and it reads the temp until about 10 minutes after driving. Sometimes I think it even causes my car to throw a code for running too lean. I have new injectors, a fuel pump, o2 sensor, and a fuel filter all new

 

So if I'm reading right it seems like you changed the thermostat right?

 

Yes bought brand new one along with a new wiring harness for it. Still having the same problems

 

How was the old thermostat when you removed it?

 

What I have in mind is maybe you installed the thermostat backwards if it even possible or maybe air in the system

 

It all slowly started with my thermostat but the old temp sensor still worked just thought it might have been faulty. It used to not do it very often at all but now it's every time I drive it. When I bought it I fixed the running too lean problem basically with replacing the whole fuel system. Purge valve, fuel pump, injectors and orings, filter, and spark plugs

 

And the old thermostat looked fine to me. Didn't seem like it looked bad

 

Unplug battery for a while. It's usually the thermostat. I just disabled the P0128 code because it is a pointless code.

 

My car doesn't give any codes. Just stops reading the temperature

 

I guarantee you if it is reading --- then it is throwing that code.

 

Just usually the --- on the dic means bad thermostat

 

New thermostat and temp sensor and no engine light or codes. I've had it read when it's not showing the temp. And the thermostat is in the correct way. If I pull my battery cable after I drive I'll get back in my car and start it then the temp reads perfect at a normal 180 degrees

 

It's very confusing I know. I've had several people look at it. Starting to think my computer might be causing it

 

did you get anywhere with this? This has been an ongoing issue with mine for a while now.. thought it was fixed with a coolant temp sensor but it's back to reading --- and making my fan turn on even on cold start.

 

Again, it is the thermostat then. Going by looks is not how to diagnose a thermostat failure. Replace with a stock temp thermostat.

 

It's either thermostat, coolant sensor or air in the system