Cooling Sensor Issues Causing Multiple System Failures?

I had a similar issue with my 2015 Sonic (same brand family as Cobalt) last summer. Got some experience working on cooling systems, but this one needed professional help. The symptoms matched yours exactly, temperature gauge acting weird, HVAC system going haywire, and sensor calibration errors popping up. Turned out the air conditioning control unit was completely fried, not just the temp sensor like I first thought. Took it to a shop and the full repair came to 980 with parts and labor. They had to replace the entire control unit since individual components weren't serviceable. The engine cooling system worked perfectly after that, and all the warning lights cleared up. Quick question, what error codes are you getting? That would help narrow things down. Also, does your temperature gauge show normal readings before acting up, or is it erratic from startup?

Thanks for sharing your experience. My error codes are P0128 and P0115, which started showing up around 89324 KM. The temperature gauge readings are erratic right from startup, sometimes showing cold, other times jumping straight to hot with no warm-up period. The temp sensor malfunction seems different from your AC control unit issue since my AC buttons and controls work fine mechanically, they just won't engage. Had my last service about 2 months ago, and everything was normal then. The engine cooling performance was fine until last week when these issues suddenly started. Really hoping it's just the sensor and not the entire control module like in your case. Will probably get it checked out professionally this week, but wanted to see if anyone had found a simpler fix first.

Thanks for those details, especially about the P0128 and P0115 codes. Actually, my situation was more similar to yours than I initially thought. After double-checking my service records, it wasn't the full control unit that failed, turned out to be a coolant leak near the temperature sensor that was causing similar erratic readings. The sensor malfunction symptoms were identical, unusual temperature swings and the HVAC system refusing to engage. The coolant leak was causing false readings, making the computer think the engine cooling system was malfunctioning. Once the leak was fixed and the sensor was replaced, everything went back to normal. The repair was much cheaper than the control unit replacement I mentioned before, around 250 for parts and labor. The P0115 code specifically points to the temperature sensor circuit, which matches what my mechanic found. Since your AC controls work mechanically, I'd bet you're dealing with the same sensor/coolant issue rather than a full control unit problem. Worth checking for any small coolant leaks around the sensor housing before replacing parts.

Thanks for that detailed follow-up. Your repair experience gives me hope this won't be too expensive. Just checked under the car and found a small puddle that looks like coolant near the front, definitely wasn't there yesterday. Seems you might be right about the coolant leak affecting the sensor readings. Going to get it into the shop tomorrow. At least now I can tell them to check the sensor housing area for leaks first, before jumping straight to replacing the whole HVAC system or control unit. Really hoping it's just the combination of a minor coolant leak and faulty sensor like in your case. That 250 repair cost sounds much better than the 980 control unit replacement. Will definitely make sure they do a proper sensor calibration after fixing any leaks. The temperature gauge acting up right from startup was the biggest red flag, and knowing your similar symptoms helped confirm I'm probably on the right track with the diagnosis.