How do I Troubleshoot the Heater on a 2002 GMC Pickup?

Start the engine and set the electric fan and heater to the highest setting to check if air is blowing across the heater core. If the fan does not switch on, a fuse may have blown, or the fan may be faulty. Have a mechanic replace the fan.

Check the fuse box (beneath the rear seat on the driver's side) for any blown fuses. Read the instructions on the inside cover of fuse box on how to locate the fuse corresponding to the heating system. If the thin metal wire inside the glass tube is broken, replace it with another of the exact same rating.

Check the carpet on the passenger-side for wetness and a strong, sweet odor. This could indicate refrigerant leaking from the heater core. The core may need to be replaced.

Ensure there is sufficient coolant in the radiator. The heating system uses a small amount of coolant to heat the vehicle. If the coolant is too low, air can sometimes become trapped and block the flow of heated air into the passenger compartment. Start the engine and feel the two heater hoses under the hood. The hoses protrude from the firewall and should both be hot to the touch. If they are not hot, the flow of coolant through the system could be blocked and the heater core may need to be replaced. The firewall is the metal partition between the passenger and engine compartments. It starts just beneath the windscreen and extends under the engine.

Check if the thermostat is stuck in the open position. Start the engine when the engine is cold. Open the hood and feel the upper radiator hose. It should be immediately apparent if coolant is flowing through the hose. If you do feel coolant, this could indicate that the thermostat is stuck open. Under normal operation, the coolant only begins to flow when the engine warms up.