I have been having this issue for a few months now, I have replaced the sensor and I checked the wiring from the sensor to the computer (major pain in the rear btw) and found nothing to cause this yet it's still happening. when the gauge is not working I get the CEL but after it starts working again a few start/off cycles clears it.

 

Hate to ask the obvious, but have you tried changing the thermostat?

 

I leave my 'real' code reader at home and carry the OBD bluetooth interface on the road. And about Rex1029 owing 6 Chevys-I don't get the importace of that statement. Not much similarity between the HHR and my El Camino, my sisters Suburban, my brother in laws 58 Impala, my nephews Avalanche, and/or my cousins Cavalier; also 6 Chevys.

 

Having seen this more than a few times, it is indeed the T-stat. Pull the code, and take a gander on here to confirm for sure. I hope you have a warranty on your HHR. I am told they are a pain in the ACE to change.

 

............ and what is the code you get when the CEL lights up?

The reason it clears up after a few drive cycles and them pops up again, is it can be very very intermittent - all dependant on how fast it warms up to operating temperature. And much of that depends on the outside ambient temperature, and how badly the thermostat is stuck in the "open" position due to the deteriorated gasket on it's internal valve seat.

 

XXXX3

 

The t-stat is actually fairly easy to change , the bolts are a huge pain in the back to get re assembled. You have to make a sticky on a socket then thread your long extension with universal joint down through the hoses and wiring,,,, the be very careful about cross threading the pot metal housing. My back has a difficult time dealing with the angles involved. A painful job, not a pain of a job.

 

So I disconnected the negative terminal to the battery for 15 minutes and voila - it works again. Like whopper said: it is very intermittent. It actually came back on its own twice in the last few days. If it were the thermostat, wouldn't it just not work at all or wouldn't the car overheat?

 

No and no, or, not necessarily.
If the thermostat is just sticking sometimes, thus you have the intermittent problem.
Most of the stuck thermostat issues seem to be because they are stuck open or partially open. In other words, they let too much coolant through and the engine can't heat up enough or quick enough.

 

I also keep my code reader in my HHR just in case, you cant be too careful,