Why Is My Heater Cold?

This is a very common question that does not have a single answer. There are several things that can cause your heater to not work properly. The object of this article is to help you to determine which part of your heater system is causing your particular problem.

First, you must understand that the heater is part of your engine cooling system. Then, it also has its own parts inside the vehicle that have absolutely nothing to do with the engine cooling system. The trick to getting your heater working is to first determine which part of the heater system has the problem.

The first step is to verify proper engine cooling system operation. There is more to this than just replacing a thermostat (like many people will probably tell you to do). If you have a problem like a blown head gasket or a defective water pump, your heater will not work. What SHOULD be done is to operate the engine while watching the computer data for the engine coolant temperature sensor (ECT). Doing this, you can verify that the engine is warming up to operating temperature. You can also verify that the thermostat is opening at the correct temperature by watching the computer data. The engine temperature will drop when the thermostat opens.

Then, use an infrared thermometer to take a manual temperature reading from the engine near the temperature sensor. The reading you get from the thermometer should be within about 5 degrees of the reading that the computer is getting from the ECT sensor.

You can also see when the thermostat opens by using the thermometer. The radiator hoses and radiator tanks should not start getting hot before the temperature of the thermostat housing is between 187 and 195 Deg. F (87 to 90 Deg. C). Checking thermostat operation in this way will eliminate unnecessarily replacing the thermostat if it is working correctly.

Also check for a clogged-up heater core by taking the temperature of the heater inlet and outlet hoses. The inlet hose should be within about 10 Deg. of the thermostat housing temperature. The outlet hose is normally 20 to 30 Deg. cooler than the inlet hose. If the outlet hose is much cooler than this, there is most likely a restriction in your heater core.

The engine cooling fans should come on between 220 and 228 Deg. F (104 - 108 Deg. C). When they do come on, the air that is blowing through the radiator should be HOT. If it is not, this is an indication that the radiator is stopped up, the water pump is not pumping correctly, or there is a blown head gasket that is filling your cooling system with combustion gasses.

If you do not have access to a scan tool that can read live engine data, all of this can be done with just the thermometer, but it is best to also verify the computer ECT data.

Please note that a pretty good infrared thermometer can be purchased at most any auto parts outlet for around $40 (US). It is a good investment because you can use it for many other things and it costs about half as much as an hour of diagnostic time at most shops. For most automotive purposes, you need one that can read from 0 to 700 Deg. F (-10 to 370 Deg. C).

If the engine cooling system is working properly, it is time to look INSIDE the car for the problem. The blend-air door in your A/C-heater housing may not be working. When you switch the temperature control inside the car, you should be able to HEAR a change in the air flow. This applies to ALL vehicles, whether they are equipped with cable, vacuum, or electronically-controlled blend-air doors. If you cannot hear a difference in the sound of the air moving through the A/C-heater housing when the temperature controls are moved from HOT to COLD and back again, then chances are pretty good that the problem is with your blend-air door.

The actual problems that can occur with the blend-air door varies depending upon what type of system your vehicle is equipped with. It could be a disconnected or "out-of-adjustment" cable on cable-controlled doors. It could be a bad vacuum servo, broken vacuum line, or malfunctioning vacuum switching valve on vacuum-controlled systems. It could be a defective electronic blend-air door actuator or a bad electronic control unit on electronically-controlled HVAC units. It could also be that the blend-air door itself is broken on any of these systems.

If your engine cooling system is functioning correctly and you think you may have a blend-air door problem, it is probably best to get it checked by a professional that has the information, equipment and knowledge to check it out. Broken blend-air doors and defective blend-air door actuators are fairly common and often require removal of the instrument panel to repair them.