I have a Spring Edition '06 HHR with 205436 miles. My check engine light came on recently with no noticeable symptoms accompanying it. As I was driving today, I watched the digital readout of my coolant temperature, and I did notice that it read hotter when the car was idle. It reached a temperature of about 212 which I don't think is bad, but normally my idle temperature levels out between 189-192. When the car is moving, the temp goes back down to about 189.

I took the car in for an oil change and to check the engine code, and it scanned a P0128, which I was told was a coolant temperature thermostat error. I just want to check in with everyone here to verify this diagnosis.

 

Most likely a t-stat, maybe a temp sender. Probably low on coolant.

No noticeable symptoms because you didn't try the A/C. And monitor your MPG.

Type "p0128" into the site search for lots of info.

 

I think the A/C and heat still work properly, I tested both while driving today. I'll keep monitoring those this week. MPG seems about the same, but I'm not certain about it.

 

If you have a CEL with P0128 the computer is supposed to disengage the AC compressor operation.
Now, if the folks that read your codes cleared them, then the AC should work normal.

As for your present issue. Maybe a little repair history info of your car should be included. Such as that heater core you were going to replace. Did that get completed ?

 

I don't think the code was cleared, the check engine light is still on. I'll test the A/C again next time I drive.

Ah yes, the heater core. I did replace it successfully with a friend's help. We put in the new heater core and hoses in August, and the heat works perfectly. No more coolant leaks, either. Overall, it was an 11 hour job. Took us only 4 of those hours to get to install the new heater core.

 

Heat and A/C work great, tested both this morning. Check engine light is still on, coolant starts getting hot when the car isn't moving. I'll plan on replacing the thermostat soon and see if that fixes the problem.

 

Oh it will fix the issue, it will

 

Just for giggles. Make sure the radiator fan is working. (AC and/or defrost off) The symptom of getting hot when sitting still can be a sign of no air flow over the radiator.

 

I would hope that the radiator fan still works. I replaced it last summer, but what the hey, I'll take a look at it anyway.

 

And that's why in post #4 I said you should include any repair history.
Especially something that's related to your coolant system.