Depending on the vehicle. There are a number of things to check. First check your fuses. You may have a bad heater control, a vaccum leak (depending on make and model of car) a short in the wiri g or a bad servo that switches from cold to hot. But first let the car get to operating temperature with the heat and fan completely of after the temp gauge goes up turn the heat on hot and the fan up. If it blows hot for a few seconds then back to cold then you need a heater core flush or heater core replacement
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