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Q: No A/C or heat and car is overheating

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At end of summer my AC started to blow warm n spurts- sometimes cold sometimes hot. If I turned the knob on dash from cold to hot back to cold it would start to blow cold again. When if started to get cooler at nite n past month I turned heat on & it was blowing cold so I played the knob game again to get heat but didn't work as well. Very random heat. I already new my egr valve plugged up & had an appt w my mechanic 2 fix that n a wk.during this wk it turned cold at night & n morn when I'd leave to go to work/school early the temp gauge would shoot up to red lined fairly qickly. Light would come on hang there mayb 10 seconds at most then gauge start to drop & light would go out & temp would straighten out. I fig was the thermostat going bad & staying closed then opening up & all going back to norm.called mechanic told him needed to add that to fix it list.. tried to open overflow (as there is no visible radiator on this car) & the cap won't come off- it's stripped.

My car has 308000 miles.
My car has an automatic transmission.

A: Hello. You may have two problems here. The ...

Hello. You may have two problems here. The engine temperature gauge behavior could be a thermostat going nuts as you describe. I would certainly check ground connections for the engine temperature sensor, and the engine-to-chassis connections in general. It may be a wiring issue.

It is possible you have a blown head gasket issue, but if so the temperature would not likely come back down very quickly at all. I would recommend having an overheating inspection completed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment and estimate for repairs. The erratic heater/AC behavior may be a malfunctioning HVAC control, or possibly a heater control valve that is sticking. Getting the engine temperature back to steady operation first will make the HVAC diagnosis much easier.

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