How to diagnose and fix a car heater not working problem
Car heater not working problems can be caused by low coolant, a plugged heater core, stuck open thermostat, or a faulty blend door. Let’s go through the checklist by symptom
Car heater not working blowing cold air
Check coolant level in coolant reservoir
If your car has some heat when you’re driving but no heat when stopped that’s usually a sign of low coolant or a failing water pump. When you’re stopped and the engine is running at a lower speed, the water pump can’t force enough coolant up to the heater core. But when you’re driving, the higher water pump speed can pump some coolant up to the heater core.
Clogged heater core
With the engine warmed up, touch the inlet and outlet hoses to your heater core. Both hoses should be the same or near the same temperature. If one hose is cold and the other hot, chances are you got a clogged heater core or an air pocket in the heater core.
Failed blend door actuator
Late model vehicles use an electric motor to move a “blend door” to adjust how much
heat is added to the air coming from the blower motor. Older vehicles use a heater control valve that controls how much hot coolant is directed into the heater core. The heater control valve can be moved by a cable or vacuum.
An electric blend door actuator can fail in any position. Locate the blend door actuator under the dash and watch the shaft when you vary the temperature knob. If the shaft doesn’t move, then either the actuator is bad or the blend door is stuck. Disconnect the electrical connector from the blend door actuator and then remove the actuator retainer screws. Move the door by hand to
see if it opens and closes easily. If it does, then chances are the actuator is bad. However, you could be facing a situation where the heater control module isn’t commanding the actuator to open and close. To diagnose that situation, you’ll need a wiring diagram and a digital voltmeter.
Most electric blend door actuators are simply reversible electric motors with a position sensing mechanism to tell the heater control module the actual position of the blend door. In this design you’ll see two wires to run the electric motor and three wires to the position sensor. Isolate the two wires to the motor and attach your digital voltmeter to those two wires. Then command a different heat setting. You should see battery voltage on your meter. If not, suspect a bad heater control module.
A blend door actuator is a fraction of the price of the heater control module and fails far more often than the control module, so if you’re going to guess at a fix, replace the actuator first.
Possible stuck open thermostat. An engine thermostat that’s stuck open will cause you to feel cold air when driving and slightly warmer air when stopped. In addition to the cold air problem, you’ll notice a drop in gas mileage.
Car heater not working blowing no air
This can be caused by a blown blower motor fuse, bad blower motor resistor, bad blower motor, bad blower motor speed switch or bad blower motor ground. Always start your diagnosis by checking the blower motor fuse. If the fuse is blown, don’t just replace it—there’s a reason it failed. If you don’t fix the root cause, the fuse will blow again.
The most common reason for a blown blower motor fuse is an air restriction caused by a clogged cabin air filter or a clogged heater core (debris on the heater core fins that won’t allow air to pass through). Remove the cabin air filter and check it’s condition. If it’s plugged, install a new filter and fuse and see if that solves the problem.
The second most common reason for a blown blower motor fuse is a failing blower motor that’s drawing too many amps. Test the amperage draw on the blower motor at all speeds. If the amperage draw is close to the fuse value, the motor is bad and should be replaced.
Car heater not working in extreme cold
If you car heater works in normal winter weather but not in extreme cold, chances are you have a failing thermostat. Do not place carboard in front of your radiator to increase heat output. That will cause uneven temperatures in the radiator across the radiator.
©, 2016 Rick Muscoplat
Posted on by Rick Muscoplat