Air conditioning works fine, but heater blowing only warm/cold air

It's an odd thing, but sometimes the air conditioner will work just fine, blowing cold air without a problem. But when heat is required, only warm air - or even cold air - is all that it will give. There are a number of things to cause this problem and looking to the heater core is not always the most obvious answer.

Recently in our forums, user "fatboy" had this problem with his 1993 Buick LaSabre. In his description of the problem, he mentions that he assumed it was the core and flushed it. He also replaced the thermostat.

First, replacing the thermostat would only be something to consider here if the engine itself was overheating (or getting hotter than normal) as the thermostat dictates when coolant is circulated through the engine. If the engine is being properly cooled, the thermostat is working fine.

Second, flushing the heater core may be a good idea if you have not done it in a while (it usually should be done when the cooling system itself is flushed). On some models, it may be a cheaper/faster fix attempt than checking teh blend door, which will be our next move.

Before anything is done, however, do the following checks:

*Check for engine error codes. Some vehicles have sensors that can tell you the blend door, heater core, cooling system, etc. are not working properly. This may help narrow the problem.

*With the engine cool, check your coolant levels. Usually, this is done by removing the radiator cap, but on some models, a reservoir connected to the radiator holds your coolant. Make sure you have it up to the "full" mark (or the overflow release inside the radiator cap seat).

*Run the car to normal operating temperature. Now, wearing a pair of mechanics or work gloves, carefully check each hose on the radiator to see if it's hot (or at least very warm) to the touch. Locate the lines going into the firewall to the heater core and check them for heat as well. All four hoses should at least be warm, depending on how long you ran the engine for - just enough to warm it will mean only warm hoses, on a longer drive first will mean hot hoses.

If any of the hoses are not warm, you may have a coolant flow problem. Reference your vehicles' manual as one hose going into the firewall is the "hot" from the engine/radiator and the other is the return. If the inlet is hot and the return is not, then you have a blockage in the heater core.

If the hoses are feeling good, then your problem may be in your vehicle's blend door(s).

Blend doors are the electronic controls that open and close small doors in your cabin air vents that allow heat to pass over the heater core (or close to block air from going in, forcing it around to keep it cool). In most vehicles, a bad blend door actuator or a stuck door will result in an error code.

Unfortunately, to get to the blend door(s) (dual climate controls will have one for each side) usually requires the same amount of work that getting to the heater core requires.