Heater in 2000 GMC Jimmy

My heater seems to be working. It blow hot air…sometimes. It will blow hot, then get cold, then hot, then cold. You get the picture. HELP. what is wrong with my car?

First check the level of coolant in the radiator. Check this when the engine is cold. Open the radiator cap. The radiator should be completely full. If it isn’t, add a 50/50 mix of water and antifreeze until the radiator is full.

If the cooling system is full, and there are no air pockets, the problem is likely to be with the air temperature flap in the ventilation system under the dash. The temp control flap sends air through either the heater core or the AC evaporator, or both, depending on the desired temperature.

If the flap is moving when it shouldn’t the air temperature will vary.

Some systems use vacuum to control the door, some use electric motors.