Why does my car’s blower motor work intermittently? (Causes and Fix) [2024]

The blower motor is a component tasked with circulating cold or warm air throughout the car’s cabin depending on your fan speed and the selected temperature.

The blower motor is controlled via the blower motor resistor and control module, which work in tandem to always provide the correct ambient temperature.

However, the system is known to sometimes blow intermittently, which is a sign of a definite problem.

The best way to solve this problem is to replace all of the faulty components after testing them out with an oscilloscope. This usually takes one to three hours to resolve.

Driving your car with a blower motor running intermittently is not likely to cause too many issues, but be sure to pay close attention to your engine running temperature.

If you want to know more about these problems which include diagnosing them and fixing them, be sure to read this article!

Why does my car blower motor turn on and off repeatedly?

The main reason why a blower motor works intermittently can be traced to problems with either the blower motor itself or its associated fan.

However, these problems can also be caused by ignition switch issues, bad relays, issues with the wiring, and problems with the resistor.

How to Diagnose an Intermittent Blower Motor

Diagnosing a faulty blower motor requires you to observe a few components and look for any signs of inconsistent operation.

Check if the blower cuts off or is slow to kick in, if there are any strange noises from the interior vent area, or if hitting the heater box turns on the blower motor.

The Blower Cuts Off or Is Slow to Kick In

While driving, be sure to pay close attention to the consistency of your blower motor. If it cuts off for no apparent reason, chances are that something is wrong.

Moreover, if the blower motor does not kick in the moment you press the associated switch, there is obviously something wrong with it.

Strange Noises Coming From the Interior Vent Area

If you hear any strange and sudden pops and crackles coming from the A/C vents or if the volume of the sound changes constantly, there is likely a problem with the blower motor voltage.

A good idea would be to crank up the fan speed to max and listen for aggressive whirring sounds which also tend to become louder the higher the fan speed.

Hitting the Heater Box Turns On the Blower Motor

If your blower motor tends to turn on and off while going over potholes and road undulations, chances are that you will be able to turn the blower back on by banging on the box/dashboard.

Some have also said that lightly banging on the engine itself can also cause the blower to turn on and off intermittently.

How to Diagnose an Intermittent Blower Motor Using an Oscilloscope

To diagnose an intermittent blower motor using an oscilloscope, you’ll need to connect the oscilloscope to the blower motor, turn the car on, adjust the oscilloscope time base, and check for ripples in the readings.

An oscilloscope is an essential diagnostics tool used by mechanics all around the world to test specific components for voltage discrepancies.

These clever tools will help you determine if the voltage provided to all of the components is as it should be and if there are any dead spots in voltage delivery.

1. Connect the oscilloscope to the blower motor.

You will first have to find the right location on the blower motor from which the oscilloscope can get a reliable and consistent reading.

The best place to do it is to connect to one of the dangling wires that connect to the resistor.

2. Turn the car on.

You will need to turn on the car in order for the blower motor to receive electrical power.

Be sure to park your car outside while doing this in order not to risk getting poisoned by carbon monoxide, which tends to build up extremely quickly in a closed environment.

3. Adjust the oscilloscope time base depending on the type of oscilloscope.

This is something you can’t ignore as the total measurements of both the duration and the required time resolution need to be taken into account if you want to determine the voltage correctly.

These are typically outlined in the owner’s manual of any oscilloscope.

4. Check for ripples in the readings.

Electrical ripples are obvious indications of electrical interferences, which typically point out that something is not getting proper voltage.

The bigger the ripple, the higher the chances of electrical interference such as a dead spot or a large spike in voltage delivery.

How to Know If a Blower Motor or Blower Motor Resistor Is Bad

A symptom of a bad blower motor or blower motor resistor is when it only works at full tilt, or in other words, it fails on lower speed settings.

The most common bad blower motor symptoms are weird noises and weak airflow through the system.

How to Fix a Car Blower Motor That’s Working Intermittently

Fixing a blower motor that works intermittently depends on what is wrong with it.

We will now list the best solutions to this problem depending on what is exactly wrong with the system.

Issue	Solution
Blower Motor Problem	Replace the blower motor.
Blower Motor Fan Problems	Use an oscilloscope to diagnose if the issue is a faulty control panel. If that is the case, replace the control panel completely. Be sure to cut out all the mating control terminals and replace them as well.
Ignition Switch Issues	Try and wiggle the key while it is in the ignition. If this affects the blower motor, replace the ignition cylinder.
Bad Relays	Consult the owner’s manual in order to locate a path to the affected relay. Once you’ve found it, disconnect the motor from the car’s main power supply and disconnect the relay slowly. Connect the new relay, connect the power supply to the motor, and test it all out.
Wiring Issues	Replace the burned wires/connectors.
Resistor Problems	You’ll need to replace the resistor. Access the resistor glove box and disconnect it from the main power supply and wiring harness. Connect the new blower motor resistor and reattach it to the wiring harness. Reinstall the glove box door. Use the oscilloscope to test out the voltage of the new resistor.

Blower Motor Problem

If the blower motor fails intermittently or dies completely, the best thing you can do is replace it as blower motor issues are notoriously difficult to repair for good.

Even if the resistor itself seems to be doing fine, it is also a good idea to replace it when replacing the motor due to potential connection wire issues that could endanger the new motor again.

Blower Motor Fan Problems

The most common blower motor fan problem is one where the control panel tasked with controlling the fan starts failing.

This means that you need to use the oscilloscope to diagnose if the issue is truly down to a faulty control panel.

If that is the case, the best thing you can do is replace the control panel completely. Be sure to cut out all the mating control terminals and replace them as well as these can also transfer heat to the new control panel and cause the same problems.

Ignition Switch Issues

Another important thing to keep in mind is that issues such as these can sometimes not at all be associated with the blower motor, but rather problems with the ignition switch.

The best thing you can do is try and wiggle the key while it is in the ignition. If this does affect the blower motor, then you should not worry about the blower, but rather about replacing the ignition cylinder.

Bad Relays

Even though this is a rare occurrence, the problem can also be caused by a bad motor fan relay. To replace it, be sure to consult the owner’s manual in order to locate a path to the affected relay.

Once you’ve found it, disconnect the motor from the car’s main power supply and disconnect the relay slowly in order not to damage any of the associated components.

Once done, simply connect the new relay, connect the power supply to the motor, and test it all out.

Wiring Issues

If the voltage being sent through the system is too high, there is a great deal of chance that one of the wires or connectors is going to get burned.

Replacing these wires and connectors is not going to solve the issue completely as you need to fix the reason that is happening in the first place.

As such, try and use the oscilloscope to find which one of the other issues listed in this article is the main reason and replace the wires/connectors if they are indeed burned.

Resistor Problems

If the resistor is the main culprit, you will have to replace it. In order to do so, you need to gain access to it via the glove box, which means that you will have to remove the glove box door as well.

When locating the resistor, be sure to test it out with the oscilloscope once more just to be 100% sure that it is indeed what’s causing the problem.

If the readings do suggest the resistor is to blame, disconnect it from the main power supply and proceed to disconnect the resistor from the wiring harness.

Now it’s time to connect the new blower motor resistor and reattach it to the wiring harness and reinstall the glove box door.

To finish everything off, use the oscilloscope to test out the voltage of the new resistor as it should now be consistent throughout the range of the fan.

Frequently Asked Questions

Is it safe to run a car without a blower motor?

The blower motor is primarily used to heat up the car’s cabin and for maintaining optimal engine temperatures. Running a car with a broken blower motor increases the chances of ending up with an overheating engine.

Moreover, you will also not be able to defrost your windows and your overall in-car comfort is going to suffer greatly.

How long will it take to replace a blower motor?

Blower motor replacement costs are typically charged at an hourly rate. You can expect the entire process to last anywhere between one and three hours in most cases.

Those that last longer than this are typically the most expensive ones, especially in rare mid-engine or rear-engine cars which place these systems in front of the engine, making them difficult to access.

How long does a blower motor in a car last?

A blower motor should be able to last the entire lifetime of the car itself, but those living in more adverse and colder climates are sure to replace the blower motor every five years or so.

Another important aspect is the quality of the blower itself as some brands use high-quality robust units that can even withstand the harshest of climates.

On the other hand, some cheaper brands use comparably cheaper units that aren’t able to last three to five years.

How much does it cost to replace a blower motor?

The national average cost of a blower motor replacement is $450 with labor included. If your car is under warranty, you might get away with just paying the labor costs, which run from $100 to $200.

The most complicated units or those that are difficult to access can cost as much as $2,000 to replace.

The cost of a blower motor replacement tends to vary greatly depending on the make and model of your vehicle and the availability of parts.
Also, the size and the horsepower of the blower motor play a great role in the final replacement costs, as seen in the table below.

Before replacing the blower motor, be sure to check the operation of all the nearby components. Some of these will also have to be replaced as blower motor problems can easily spread to some of its nearby electrical components.

You can also save some money by going for a used blower motor unit from a car with not too many miles on the clock as these tend to go for under $80 with little to no wear.

Motor Horsepower	Motor Price	Price and Labor Costs
1/4 HP	$100 – $500	$300 – $900
1/3 HP	$100 – $800	$300 – $1,200
1/2 HP	$120 – $1,000	$320 – $1,400
3/4 HP	$175 – $1,500	$375 – $1,900
1 HP	$250 – $2,000	$450 – $2,400

Written by Haim Levy

Haim Levy is an automotive expert and technician with over 29 years of experience. His love affair with cars started at an early age and only deepened after he went to Sheridan…