Q: When A/C is on, the car overheats

Engine overheating with AC on is generally caused by one of two possibilities. One, is increased engine load caused by a failing AC compressor. The AC compressor adds rotational load to the engine and if the compressor is wearing and becoming increasingly harder to turn this can cause engine overheating.

The second common cause is a cooling system not working efficiently. The AC condenser heats up as the AC is being used so the engine cooling system has to deal with this increased heat. Plugged or blocked radiator condenser fins, fans not working efficiently or a water pump that is not circulating coolant could all cause overheating with AC on only.

One last possibility though more unlikely as the vehicle would generally read hot whether the AC is on or not is a faulty engine coolant temperature sensor. The coolant temp needs to be read out with a temperature gun and needs to compare with the gauge. The radiator and condenser fins should be cleaned of debris. If these items do correct the issue, then you may want to enlist the help of a mechanic, such as one from YourMechanic, who will have the expertise to diagnose and correct the overheating issue.