The heating system in your car does more than just keep the interior warm in cold weather. It keeps the engine cool by ventilating the engine heat elsewhere. Fixing any problems with your heating system is often best left to a professional, but there are some things you can fix and replace yourself. The key components in the heating system that might need replacing are the engine coolant, heater core, hoses, thermostat and the drive belt along the compressor.
Drain the engine coolant before working on any part of the heating or air conditioning system. With the parking brake on, remove the drain plug from the coolant tank and let the coolant pour into a clean container. If the coolant looks old, discolored or contaminated, refill the car with fresh coolant.
Change the drive belt for the air conditioner compressor if it is worn down. Rotate the tensioner to relieve the tension (the direction you rotate it depends on the car), and slip the belt off. Rotate the tensioner again to fit the new belt on its components; it will retighten once you release it.
Replace any heater hoses that are dry, cracked or leaking. Many of these hoses have quick-connect fittings that are released by pressing tabs to remove them.
Clean any debris or bugs out the condenser by blowing compressed air into it.
Replace the thermostat if it is stuck open or closed. This is usually contained in a housing on the upper radiator hose near the engine. Remove the housing by loosening its bolts, and take note of the thermostat's exact position and orientation as you remove it. Use a new O-ring or gasket with the new thermostat.
Flush the heater core and/or heater hoses with a running garden hose if there is a blockage. This is a likely cause if one of the hoses feels cool while the heater runs. Disconnect the hoses at the firewall to flush the core and remove the hoses completely from the engine to flush them.
Remove and replace a bad or leaking heater core (one sign of this is coolant leaking onto the floorboards). The housing that contains the core is often right behind the instrument panel, meaning that you must disassemble and remove the dashboard to reach the core.
