Why does my car overheat when the AC is on?
Running the AC adds heat load in front of the radiator and extra drag on the engine, so a cooling system with no margin left overheats with AC on and looks fine with it off. The prime suspects are the cooling fans — most cars run them at high speed whenever the AC compressor is engaged, so a dead fan, bad relay or failing fan clutch shows up here first. After that: a condenser and radiator stack packed with debris, a partly clogged radiator, a weak water pump, or a thermostat that's slow to open. Turn the AC off and the heater on full if the gauge climbs; that buys you the drive to a shop.
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