The heater not blowing is no joke, I lived and worked as a mechanic in Canada for a time, and nothing could have prepared me for the winters. A good working car heater isn’t a luxury; it’s critical to staying alive.
Top 4 reasons car heater blower stops working:
- Heater blower fuse blown
- Failed blower motor
- Faulty blower motor resistor
- Failed blower motor relay
In this post, we’ll cover all the common causes of the blower motor failing to work; you’ll learn how to diagnose your fault and how to fix it.
1 Heater Blower Fuse Blown
A blown fuse is by far the easiest item to check, so it’s first on the list. Fuses usually blow for a good reason; while it is true that colder weather can cause greater amp draw, which can cause fuses to blow, it is more common for the fuse to blow because of an underlying issue.
Modern cars carry a ton of electrical consumers who all need a fuse; the point is your vehicle will have a ton of fuses in two or more fuse boxes located inside and under the hood.
A quick Google of your make and model will identify the fuse box location and fuse number; alternatively, look in the driver’s manual under the fuses section. Most models will list the fuses on the rear of the fuse panel cover, which is super helpful; I wish they all did it that way.
How to check the blower motor fuse?
With the fuse box located and the blower motor fuse identified, go ahead and remove it and check the filament. If it’s broken, the fuse is blown. Replacing the fuse may solve the problem, or you may find it blows again either immediately or shortly thereafter.
The blower motor pulls a lot of power when working, about 35 amps; the fuse will likely be a large amperage of 40A or so. Fuses are color-coded with the fuse size also embossed into the casing.
A common cause of fuse failure fits a fuse that’s too small for the job. It is important to replace the fuse with a correct amperage; too small, and the fuse will blow again, and too large could cause a fire risk.
If your fuse is blown and you replace it only to find it blows again, you may have a blower motor restriction (check for tree fall in the fan blades) or a short in the wiring, or, more likely, a faulty blower motor. More on faulty blower motors later.
After checking the fuse, an auto technician would reach for a code reader and check the car’s climate control module (computer) for fault codes. This turbocharger is the whole diagnostic process; if you have one, now’s the time to plug it in. You can check out my favorite bidirectional tool on the Auto electrical repair tools page.
As most don’t have a scan tool, I will continue to diagnose this with just the aid of simple tools.
2 Failed Blower Motor
Blower motors work hard, and failure is common. A slow start, intermittent operation, or squeaking noises are all early signs the blower motor is failing.
Some common causes of failure include blocked windshield cowl drains cause rainwater to access the heater assembly, and damage to the motor windings. Fan restriction is another common cause of motor failure. The blower motor sucks air from outside the cabin through the cabin filter. If the filter is missing or damaged, debris such as tree fall can restrict blower movement.
The blower motor is commonly located passenger side front under the dash panel. Access may be a pain in the jacksie, lay on your back with your head in the passenger footwell. Some cars are easy, you look under the dash, and there it is. Others, usually higher-end models, will require several panels and possibly glove box removal.
How to test the blower motor?
Testing the motor is a simple process. We will need a voltmeter or basic test light. The blower motor is located passenger side front under the dash panel. With the ignition and fan speed set to high, try the wiggle test; wiggle the wire to eliminate a bad connection.
Assuming the wiggle test didn’t work, go ahead and remove the two-pin blower motor connector. One wire is power, and the other is ground. Check both with the test light (move the test light crocodile clip to power to test ground side). When testing with a voltmeter, it should read 12 volts.
Three readings are common:
- Power and ground test good (1 and 2 above) – Blower motor has failed; go ahead and replace the motor.
- Ground side is missing – Check the resistor/blower controller (see below).
- Power side is missing – Check the resistor and fan relay for fault (see below).
See the Auto electrical repair tools page for a voltmeter, test light, and other helpful diagnostic tools.
3 Failed Blower Motor Resistor
The heater blower motor resistor is a small controller that’s employed to reduce the speed of the blower fan as per requests from the driver. The resistor reduces the voltage supply to the blower motor, which slows fan speed. Modern cars use a blower motor controller, which is a resistor by another name.
Resistors work hard to reduce voltage; they are, therefore, a common cause of blower motor faults. Depending on the design and how they fail, resistors either stop the fan from operating or cause it to only operate at full speed, no matter the fan speed request.
The resistor is commonly mounted to the fan housing near the blower motor fan. As the resistor reduces voltage, it produces heat as a by-product. To dissipate the heat, resistors employ a heatsink (cooling fins) and utilize the cool air from the fan speed to help cool the resistor.
How to test the blower motor resistor?
We will need a voltmeter or basic test light. The resistor is located as said on the blower motor housing, and the blower motor housing is commonly located passenger side front under the dash panel.
Resistors commonly use four or five wires, a power input, a ground input, a control signal input from the heater control panel via the heater control module, and a variable ground output to the blower motor.
The variable ground output is what controls fan speed (some may also supply power to the motor). Note your wiring may be different, but the general working principle is the same.
Checking the resistor is like any other controller; check power and ground, signal inputs, and outputs.
- Check resistor power and ground inputs (1 & 2 above) – With ignition on and the fan turned to full, remove the blower motor resistor connector. Four wires are common; check ground and power inputs. If power is missing, check the blower motor relay (see below).
- Check resistor output – With ignition and blower motor on and test light crocodile clip on a good power, probe the ground while turning the blower motor control on the control panel. The test light should vary from bright to dim etc. if it doesn’t, go ahead and check the resistor input.
- Check resistor signal input – With the resistor unplugged, use a multimeter to check the voltage between the power input and the signal input while varying the blower motor speed. Voltage should also vary; if it does go ahead and replace the resistor, it’s faulty. If the voltage doesn’t vary, your control panel, climate control module, or wiring between them is likely at fault.
4 Faulty Blower Motor Relay
Relays are employed to safely control a higher amp circuit, such as the blower motor using a low amp circuit, such as a heater control panel. Relays are small electromagnetic units that are commonly located inside the fuse box. Relays are generally durable, and that’s why they are last on their list of items to check.
Most relays are four terminals, a load side (1 (30) and 5 (87)) and a control side (2 (86) and 4 (85)). The control module operates the control side and, when activated, causes high amp power to flow through the load side of the relay.
How to test the blower motor relay?
Locate the blower motor fuse, likely a 30 or 40 amp; it may be marked on the fuse box cover or in the driver’s manual.
Use a test light or voltmeter to check for power at the blower motor fuse with ignition and blower motor on.
Fuse boxes contain lots of relays; many are identical and can be interchanged. A fast way to eliminate a faulty relay is to swap out an identical one and test the blower motor; just be sure the relays are identical.
Check out the inexpensive relay tester on the Auto electrical tools page.
Another quick test is to listen for a click when the blower switch is turned on. Relays give off an audible click initially when powered up. The click is the electromagnetic arm activating.
However, hearing a click doesn’t mean it’s working correctly; it simply means the circuits appear to be good.
Using a multimeter, test for power at the fuse with ignition and blower motor set to on. If power is present, all is good with the relay.
If, on the other hand, you have no power at the relay, that suggests a fault with the relay or the controller.
If swapping out the relay doesn’t help, go ahead and check for power and ground on the control side using the multimeter to help identify where the fault lies on the load side of the control side.
A lack of power on the control side suggests a problem with the control module. Control modules are expensive and may require software upload and calibration after installation. If you suspect a fault in the control side
Check out my favorite wiring repair tools on the Auto electrical tools page.
- About the Author
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.