I have a 2009 Chevy Malibu lt 2.4L and the temp gauge will drop while driving or it wont even read that the engine is at operating temp but the ac blows cold air and I got a scan tool and no codes were tripped so i hit reset all codes and the needle went up to operating temp and I replaced the coolant temp sensor any clue what it could be
First check to ensure you have enough coolant. Air pockets around the sensor will cause erratic readings. I've never seen a code get thrown for lack of coolant in the radiator. If it's low, check for a leak.
If it's full, do a pressure test to ensure it's holding the pressure it should. My next guess would be a faulty sensor but you've just replaced that. Check for damaged wires leading from the sensor to the wiring harness.