Car AC Compressor Won’t Turn On? 8 Reasons and Solutions

As an amazon associate motorhills.com earns from qualifying purchases.
Car air conditioning

It’s really frustrating when you flip your car’s AC on and you still get pummeled with hot air. During a hot summer day in the South, this is enough to drive you insane.

If the AC compressor doesn’t turn on, the most common culprit by far is low AC refrigerant. If there is a leak, no matter how small, it’ll eventually cause a non-working AC system. Another possible issue is a blown fuse and/or relay. Your owner’s manual should be able to tell you where those are located. Poor grounding, frayed wiring, bad clutch coil, and defective AC pressure sensor switch can also be an issue although less common.

The good news? Hot air blowing from your vents isn’t a death sentence for your car. It could be the AC compressor not turning on. In this guide, we’ll give you 8 reasons why your car’s AC compressor isn’t turning on along with some solutions to each problem.

What Is an AC Compressor?

The compressor is just one part of your car’s HVAC system. A lot of people don’t really think about how that cold air gets cranked out of your car’s air ducts even when it’s 100 degrees outside.

It takes a little bit of magic and a whole bunch of science.

AC-compressor
The A/C Compressor will normally be at the front of the engine with the other belt-driven units.

Your car sucks in air from outside as you drive along. The air goes through a process that takes all the heat and humidity out of it. The result? Cold air coming from your vents.

One of the big culprits is your AC compressor. There’s a gas vapor called refrigerant that does all the magic of taking the heat out of the air.

The compressor circulates this refrigerant around. Once the refrigerant sucked up its fair share of heat, the compressor moves it out of the way so “fresh refrigerant” can take its place.

If the compressor doesn’t do this, the air won’t be sufficiently cooled as it passes through your HVAC system and out of your vents.

Automobile air cooling system.
The air cooling system in the car cabin is primarily used to remove heat from the cabin, using the compressor and clutch plate device to working start.

8 Reasons Why Your Car AC Compressor Won’t Turn On (and Solutions)

There are a number of reasons why your AC compressor might not be working. Each reason’s solution is a little different. Let’s take some time and explain all 8 reasons why your car’s air conditioning compressor won’t turn on.

Low Refrigerant in the AC System

This is by far the most common issue as to why your AC is not working. Low amounts of AC freon refrigerant will be detected by the AC pressure sensor switch, causing the AC not to kick in.

The most common refrigerant used today is the R134a refrigerant. There are two pressure states that need to be checked. As seen in the pic below the low-pressure is represented by the blue whereas the high-pressure side is the red one.

When using AC manifold gauges to check for good pressure, check both high and low. On the low side, there should be around 30-40 PSI at 75 degrees ambient temperature outside. If it’s on the low side, this may be an indication that there is a slow leak.

The on the high-pressure side, the PSI should be twice the ambient temperature. So for example, at 75 degrees outside, the high pressure should be between 150-170 PSI approximately.

Solution: After testing the AC for the amount of PSI in both low and high-pressure lines, re-fill the AC and monitor the gauges carefully. Some AC refrigerants already come with the gauges built-in.

If you suspect there is a leak when re-filling the AC refrigerant, a dye can be used so that next time it stops working, the dye will be visible when you check it in the dark with a UV light.

Any leaks will glow in the dark (greenish-glow). I would recommend starting at the A/C compressor and going along the A/C lines inspecting all areas carefully.

AC freon refrigerant refill
Re-filling AC freon refrigerant

A Faulty Car AC Pressure Sensor Switch

When you click the dial on your center console to “cold” and nothing happens, it could be thanks to a busted temperature control switch.

This switch toggles your HVAC system from hot to cold. If the switch isn’t working, then your car’s HVAC will be stuck on one of the two settings.

In your case, it’s stuck on the heat setting. Your compressor might be fine, it’s just not getting the signal to get things chilly.

The only problem is that it’s not easy to test this control switch. Your best bet is to try some other things on this list and if it still doesn’t work, then replace this temperature control switch.

Solution: Replace the temperature control switch.

AC pressure sensor switch
AC pressure sensor switch

Relay or Fuse Problem

Capacitors and relays are little electronics that are built into the wiring around your car. Without getting into the electrical intricacies, we should tell you that these pieces are imperative.

If a relay or capacitor has a problem, the full voltage won’t make its way through the full journey. All of the voltage needs to make it to your compressor or else it won’t run.

Solution: Inspect the capacitor and fuses for the A/C, replace them if they look worn or damaged.

Car fuse box closeup. Multiple rows of different fuses, one connector and part of a relay visible.
Car fuse box closeup. Multiple rows of different fuses & relays

Compressor is Dead

The simple fact is that sometimes the compressor is just dead. A component inside of it might have broken, leaving it powerless.

Once the compressor is dead, the only solution is to completely replace it. This is something that you can do on your own so you don’t have to take it to a mechanic. It might be a couple of hundred bucks in parts if you choose to tackle this project.

Solution: Replace the compressor.

detail of the engine  fragment include ac compressor belt and pulley.
Inside the engine bay – A/C compressor mounted to the engine with belt installed

It’s Not Getting Power

The compressor needs to get voltage before it can turn on. If this wasn’t the case, your AC would be blowing even when the car was turned off.

That means that if the vehicle has low voltage, it can affect the electronic modules that affect the air conditioning system. Once the vehicle is running, the alternator will provide constant voltage, but there could still be some error codes.

Solution: Troubleshoot your battery. If it’s getting below 12 volts, it could be a faulty battery. Try jumping your car, or check if the battery needs to be replaced.

Inspecting the car battery
Inspecting the car battery to ensure it’s getting at least 12v

Wiring Issue

Yet another electrical issue that can arise is improper wiring. This doesn’t mean that the compressor manufacturer crossed some wires, it means that something went wrong along the way.

If your compressor never worked, then it could mean that the internal wires weren’t installed correctly. If you just recently installed the compressor yourself, make sure you ran the wires correctly.

If this problem started happening randomly, you might be able to thank mice. They are known to chew through wires to sharpen their teeth. You’re left with disconnected components all over the place.

Solution: Check for damage to wiring and ensure all connections are tight.

Large wide cable with multicolored red and green wires and conne
Visually inspect the A/C wiring going to and from the A/C components (Fan clutch, compressor, fuse/relay) in the engine bay to the other side of the firewall where the ECU/BCM and climate control are. Sometimes wiring can get corroded, frayed, or be completely severed.

Compressor Clutch Isn’t Engaging

The compressor shaft will rotate when it’s turned on. There’s a little clutch that then gets pushed against the shaft and engages. Once the clutch is engaged, the air will start cooling down.

If the clutch doesn’t engage, then the compressor will just freely spin with no reaction.

There’s an easy way to troubleshoot this. Turn your car’s HVAC setting to “heat” to blow hot air. Pop the hood of your car and locate the compressor. You should notice the disk on the pulley side of your compressor is not moving at all. This is the clutch and it should be disengaged when your car is running the “heat” mode.

Get in the car and turn it to AC and go back to the front of your car. Check to see if the clutch is moving now.

Solution: Check the connections on the clutch and replace the clutch if there’s mechanical damage to it.

AC-compressor-clutch-highlighted

Rusted Clutch Plate

Let’s start by saying that rust on certain parts of your clutch plate isn’t going to cause your compressor to break. It’s actually the byproduct of a compressor that has seen better days.

Rust on the outer ring of the clutch, however, could be the culprit. In either case, if you spot rust, it could be an indicator that your compressor died from old age.

Solution: Replace the clutch plate and potentially replace the compressor, too.

Prevent a Dead AC Compressor in the Future

As you can see, there’s a lot of troubleshooting that goes into fixing an AC compressor that won’t turn on. If you want to skip this laborious troubleshooting, you should know how to prevent this problem.

The biggest means of prevention is maintenance. You might want to get an AC tune-up from a car shop every spring so you can get ahead of the problem.

Mechanic with manometer inspecting auto vehicle air-condition co
Mechanic inspecting the vehicle’s air-conditioning compressor with a manometer.

Besides that, it’s a good idea to run the AC for 10-15 minutes once a month, regardless of how cold it is outside.

If you spend a little time and money today on maintenance, you can save a lot of money tomorrow. This is one of many ways to ensure your car lasts forever.

Conclusion

We hope that you find the root cause of your AC compressor not turning on. Our 8 reasons and solutions should help you narrow it down and troubleshoot your way through this issue. For more car maintenance tips, explore our blog. Make sure you have the right accessories and tools to make things easier.

Your subscription could not be saved. Please try again.
Got it! Check your inbox for a confirmation email.

Sign up for Exclusive Car Tips

Get Access to Useful Automotive Tips from Motor Hills

Motor-Hills

Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

Leave a Comment