Do not replace thermostat unless you're sure it's defective. It's a lot of wasted work if it isn't. If the gauge NEVER comes up from zero, it's not the thermostat.
With a thermostat stuck open it will take a long time but the engine will eventually come up to temperature so the gauge will eventually go up. You can speed it up by driving the car or blocking the grille.
I'm with rebel65, you probably have a bad connection - loose or damaged wire in the connector at the ECT. Handling the connector during the ECT replacement probably got it re-connected temporarily. So inspect it closely. Pull back the loom and and rubber sheath below to get a good look at the wires going into the connector.
Worst case, you can get replacement connectors with wires attached (ie, pigtails) and swap them in for the last few inches going to the sensor. Check Rockauto, most parts stores, and eBay as well.
BTW, dxG6 is dead nuts on - no need to touch the thermostat.
I have an 07 Malibu with the same problems. I’ve come to believe it’s the ECM. When your ac isn’t working try disconnecting the battery for 10 seconds and see if it resets. If that works it rules out the thermostat being stuck open. And I’ve changed out thermostats and temp sensors a few times and it’s only a temporary fix.
Good luck and let us know if you find anything.
I've had the trac/esc light come on randomly about once every couple of weeks for a while. It goes off at the next key-on and car works normally. Has anyone seen this and found the cause?
I did re-grease the BCM connector, that didn't fix it.
Hell everyone i have a 07 pontiac g6 gt convertible and i recently hit a coyote coming home from work and had to replace radiator cooling fan and ac compressor and i got it all fixed but now my radiator line keeps pooping off and i cant find out why and i leak transmission fluid everywhere and...