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In August I noticed my AC was blowing hot air and the check engine light came on throwing the p0128 code. I replaced the thermostat and the AC and coolant temp on the dash were working for a few days before both stopped working and the check engine light came on again, throwing the same code. I let it sit for a few months while I was at school and I came back and replaced the coolant temp sensor. The check engine light went away and the coolant temp was reading, but once it reached about 117 degrees the reading went back to ---F.

I have read a lot of forums saying that this is a common Cobalt problem and I have replaced the recommended parts, but the issue remains. What else could be the problem? Faulty wiring? Heater Core? Any suggestions?
 

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I would probably try to test things first. But sounds similar to a bad control module given the fact you have replaced parts that should have fixed the temperature issues. Along with a non working AC. Again test first.

Make sure it’s not a coincidence that the AC isn’t working. I’d go about testing the AC refrigerant pressure first to make sure it didn’t just leak out. The high side valve is a know problem on GM vehicles due to its poor sealing design. They can just randomly start to leak. If that’s the cause it could have caused enough refrigerant to leak out and cause the system to no longer cool.

The temp reading is many times a faulty thermostat. Sometimes the cheaper ones won’t work correctly and you’ll need to buy the original AC Delco part or a premium part to replace it with. Sometimes the ones that say premium are crap as well. It can happen that a new thermostat fails early. But not the highest of likeliness unless is a cheap one.
 
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