Why Doesn't My Car Heater Work?

A. Check Engine Coolant Level - Most cars have coolant reservoirs with opaque white plastic tanks and measuring lines allowing you to see if the coolant is at the recommended level. Coolant expands when heated, and for that reason, most coolant reservoirs have a Full-Cold and a Full-Hot line.  If there is too little coolant in the reservoir, you should be alerted to the possibility that there could be a leak somewhere in your car’s cooling system.  Remove the reservoir cap and add coolant to the Full-Cold line, if a very short time after you refill the coolant reservoir the coolant level is low, there is a leak in your car’s cooling system. Engine coolant is used to heat the heater core and if the vehicle is low on coolant the heater core will stop working. The heater cannot produce heat due to the lack of heat from the engine coolant. Even if the engine is not over heating the coolant level can still be low. Inspect the engine coolant level in the coolant reservoir tank; coolant level should be between the hot and cold marks. Always check the coolant level when the vehicle is cold, preferably over night. Check for engine coolant leaks and repair as needed, then refill with coolant and recheck the system.

Radiator Cap and Coolant Reservoir

Heater Control Valve

D. Trouble Shoot Heater/Air Conditioner Blower Motor.  An electrical motor is used to push air through the heater and air conditioner system. If this electric motor fails it will not force air through the system. Ground a test light lead to a good ground source, like a metal brace under the dash, seat mount bolt or under hood brace. Turn the key to the on position; check the fuse in the heater or blower motor fuse panel with a test light. Then lightly touch the service port at the top of the fuse on both sides. If the test light illuminates on both sides of the fuse, the fuse is ok and working properly. If the fuse lights the test light on one side it is blown and needs to be replaced.

 
Power Distribution Center

If you are unsure of the location of the heater fuse consult a repair manual. If the fuse condition is ok, position the blower speed to the highest setting. Connect the ground end of the test light to a known ground source. Then probe both ground and power feed wires of the fan motor. The test light should illuminate on only one of the wires (fan motor failed). If the test light illuminates on both wires the ground source has shorted. If neither of the wires illuminate the power source is shorted. To trouble shoot this style of problem a wiring schematic is needed.

Related Subjects

• If Your Engine is Low on Coolant - Many points in your car’s cooling system are liable to develop leaks through which coolant can escape. To inspect for a leak in your car’s cooling system, you should understand that the main physical components involved in the cooling system are 1) the engine block; and then, going forwards towards the radiator 2) the thermostat; 3) the upper and lower hoses leading respectively into and away from the 4) radiator; 5) the radiator cap; 6) the radiator fan; 7) the water pump and, going from the engine block towards the passenger cabin; 8) the heater core and its associated hoses and fan. A leak in the heater core most often results in escaped coolant on the front passenger-side floor of the passenger cabin.  Most automobile antifreezes are either green or red and despite a sweet smell they are very toxic.  Particular care must be taken to dispose of old coolant properly, as the sweet smell can attract children and pets, and disposing of old coolant at random contaminates the environment. The best repair for a leaking heater core is to replace the core. Visible signs of leakage are the most obvious indicator of heater leaks, but you should also physically inspect the heater core for signs of deterioration.  When checking the heater hoses squeeze one hose at a time, you should feel that the hose is soft and flexible.  There should not be any irregular bulges in the hoses and you should not feel any dry rot, which manifests as brittleness and/or cracking.  You should not feel any weak spots in the hoses. If you detect any of these conditions, replace the heater hoses. When replacing heater hoses measure the length and diameter of your old heating hoses so you will know the size of the new ones.  Most cars use two different sizes of heater hoses; the standard diameters are 1/2”, 5/8”, and 3/4”.  When removing the old hoses, take care not to use any yanking or turning motion that could damage the heater core.  Mechanics often score the old hoses open with a knife and then gradually coax them loose before removing them.

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