I have a 2010 1LT HHR with a slushbox. When the car idles while sitting in traffic (which is mercifully not very often here in Tulsa) or sitting at a drive through, the temperature gauge goes higher (but I've never seen it go into the red) so I guess you can say it doesn't get hot, it just gets warm. I always shut it down when it gets too warm so I don't know if it ever gets into the red. As soon as I drive off, the gauge goes immediately down to about 1 mark before the thick "middle" line. At first I thought it was a bad fan, but the fan still runs when the car gets warm. Any ideas? Could it be the thermostat?

 

yes it is the thermostat. it is closed and there heating

 

Check the engine temp reading on the DIC by toggling the "Info" button on the steering wheel, HHR's depend on "Ram Air" through the nose for cooling while they're moving.

187-192° is the normal temp range while under way, idling even in winter they can creep up to the 217° mark if you're stopped, at that point the temp sensor sends a signal to the ECM commanding the cooling fans to start.

When the fan kicks on, you'll see the temp drop back to 187-192° again.

If the temp readout on the DIC says "--", or if the engine goes over 230°, you may be dangerously low on coolant or have a duff thermostat.

The temperature gauge is one of those "approximate" deals, the temperature readout on the DIC is very accurate as is gives readings fed from the ECT Sensor.

 

Let's see!
A 2010... 5 years old!!
Time to replace the thermostat ( please save yourself the expense and aggravation and get an AC Delco and use Dexcool, these Trucklets Ecotecs are built for it!
The aftermarket t-stats don't open within the time frame the ECM is programmed for which can cause codes , issues with overheating and other havock.
Please park the HHR with the grille end higher than the rear end!
This will help burp the air out when you refill the rad!