Hey John, I never finished was going back and forth with youl. I haven't had time to complete the work yet but I was able to get the Coolant Temp Sensor. I'm trying to install the new one but need to know if I have to drain the cooling system and if I need some sort of sealant to go around the threads of the sensor.

Check your coolant level first, then the heater control valve that diverts engine coolant through the heater core. There's a few other things that regulate heater temp in your car.Found a site that may be of help to you -
2013 Chevy Sonic no heat DoItYourself com Community Forums

Hey Rayna,,, if you are sure the coolant is not circulating..??.. Then the water pump is no longer functioning.. OR....the coolant passages have partially clogged up.. When you replaced the thermostat , you should have reversed flushed the entire coolant system, check the radiator hoses for wear (they wear out from the inside), flushed the radiator completely..... Do the complete flush and check the hoses.. refill and see how much better it does..

Hey Rand,
When the engine gets hot enough to stop the A/C from working.. you do have some issues,.. When the thermostat was replaced, was the entire coolant system reverse flushed and cleaned out? If no, its time for a full reverse flush and clean, new thermostat and coolant!

The code P1245 is for the Intake Plenum Switchover Valve.

This probably has nothing to do with the gauge fluctuations. The gauge problem is probably one of three possibilities:

1. Low coolant level in the system. When coolant levels get low, air moves thru the system and when it reaches the temp sensor, will cause fluctuations.
2. Defective coolant sensor. These can fail over time as do all sensors.
3. Defective gauge in the dashboard.

could be a hose but it is costly
always check the water and have it pressure tested fast
driving with a leak can empty the sustem and burn the engine

best case the hose blew out and you lost coolant causing it to over heat. and could be repaired with new hose. worst case when an engine with aluminum cylinder heads overheats it can cause the aluminum heads to warp and crack making cylinder head replacement necessary

Low freon for the summer, and low coolant for the winter. The heat and cooling are not related to each other. Separate systems

Timing belt/chain may have been put it slightly off time, when motor rebuild

check coolant level in overflow bottle and the radiator first.

check to see if temp blend door is rotating back and forth. Rotate temp knob while watching the motor that operates the blend door under the dash.

check to see if both heater hoses are hot where they go into the cowl in the engine compartment into the heater core. If not hot, check temp gauge to see if it reads nomal, if not replace thermostat.

hi john , sure sounds like you may have unplugged the coolant temp sensor or didn't get a good connection at the sensor , location , in between two exhaust ports on the side of the engine that has the oil fill and pcv .

J5