The car overheated out of the blue but when I turned on the heater, the temperature came down.
What might the reason my engine is overheating? on 2004 Volkswagen Passat
start with the easiest first, check your coolant level and top it off if needed. then let your car idle until it comes up to operating temp and CAREFULLY check your coolant level again. if it is low you will have to examine all hoses and connections for leaks. after that the other possibilities are a bad thermostat or a radiator fan malfunction. describe the conditions at the time; stop and go traffic/ long periods of idling/ air conditioner on or off/ etc.
ck fluid level and fan operation to start with