I have a Mazda 626 year 2001 my car overheats with the heat on but it runs at normal temp with the air on?
If it runs normal temperature with the A/C on, this tells me the electric engine cooling fan is operating properly in A/C mode. The overheating in heater mode makes me think there is a problem with a temperature sender or fan control unit. When the engine gets to a certain temperature in heat mode the electric fan should come on. When you are in the A/C mode the fan will run all of the time.
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