How to Tell if a Car AC Compressor Is Bad (7 Signs)

AC compressor belt

Your car’s AC compressor is the driving force behind the cold air that comes out of your vents. As you’ll learn in a little bit, your AC won’t do anything if the compressor is bad or dead. This can lead to some uncomfortable drives during a Floridian summer, so I’m here to help.

If your car’s AC compressor isn’t working, you’ll be getting warm or hot air coming from your vents when your AC is on. You might notice refrigerant leaking or hear a weird noise from the compressor as you turn on the AC. If there is an electrical component failure or the clutch isn’t engaging, your compressor won’t turn on.

In this guide, I’ll talk about your car’s AC compressor. I’ll give you 7 signs that might indicate that it’s bad, I’ll teach you more about how it works, and I’ll give you tips for repairing it.

What Is an AC Compressor?

The compressor is a part of your car’s HVAC system. As a general overview, the HVAC system takes incoming air, cools it down, then blows cold air through vents in your car.

The incoming air is cooled down by passing it through refrigerant. The refrigerant absorbs the heat, then needs to be pumped away, turned from a gas to a liquid, then recirculated so the process can continue.

Close up of a brand new AC air conditioning compressor against a grey background

This is where the compressor comes in. Your car’s AC compressor pressurizes the refrigerant and pumps it away to the condenser. To put it more simply, the compressor is there to keep the refrigerant moving around the system.

If the compressor is off, then the refrigerant is not cycling through the AC loop, and it’s not cooling down the air. This is great news if you’re driving in the middle of winter and want warm air. However, it’s bad news if you need cold air to be delivered. It’s even worse news if you have your AC system turned on, and the compressor isn’t firing.

7 Ways to Tell if a Car AC Compressor Is Bad

Now that you have a better understanding of your car’s AC compressor, I can get into some diagnostics. This section is all about finding out if your AC compressor is bad. I came up with 7 ways that professionals use when they’re troubleshooting the same issue.

1. Hearing Weird Sounds

In the automotive world, any abnormal sound is a bad sign. The only thing you should hear while you drive is your massive engine, the wind blowing by, and your favorite AC/DC song on the radio.

If you turn on your AC and hear a humming, click, grinding, or groaning sound coming from the front of your car, that’s not good.

Car mechanic inspecting the engine up close checking for any issues

Any of these sounds can be an indicator that something is wrong with your compressor. The compressor has some bearings inside that allow things to move around and generate motion for the refrigerant. If a bearing is bad, then the rotation will include some type of interference with a shaft inside of your compressor, hence the weird noise.

A click with nothing else means that the compressor isn’t firing in the first place. A groaning, grinding, or humming noise means that it’s likely on its last leg. It needs to be replaced as soon as possible, and the performance will keep getting worse until it finally stops working.

2. Vents are Blowing Hot Air

One of the biggest signs that something’s wrong with your AC is if you’re getting hot air from your vents. Admittedly, this could come from a number of different failing components, but the compressor is one of the biggest ones.

A person holding their hand in front of the car's dashboard vent checking if the air conditioning ac system is working properly

It might also mean that you’re low on refrigerant, which is a quick and easy fix. Before the compressor finally dies, the air will get warm, and you’ll have a chance to fix it. As the compressor continues getting worse, the air will just get warmer.

If you feel warm air coming from your vents when the AC is turned on, then it’s time to start troubleshooting.

3. Fluid Is Leaking from the Compressor

Besides noise, another good hint that there’s a bigger problem is if fluid is leaking. Since I’m talking about the compressor, the fluid in question is refrigerant.

If you remember back, I explained how refrigerant is the magic fluid that makes air cooler as it comes through your car. Without the refrigerant, there is no cooling.

The reason why a faulty compressor results in leaking fluid is because the connections are failing. There are bearings and fittings that are used to keep the refrigerant flowing through the compressor and to the rest of the system.

If any of these connection points become loose, damaged, or fail, then fluid will leak out of your car.

Hooking up a machine with pressure gauges PSI up to the AC air conditioning system to ensure it's working normally and checking for leaks

It’s likely that the fluid is coming from the compressor because of all the extra pressure in this mechanism. Since the compressor is squeezing refrigerant, it’s always in a pressurized state. Continual exposure to higher pressures means a higher chance of failing, hence the fluid leaking from your compressor and not a random HVAC hose.

I should also mention that it’s common to find fluid on the ground under your car when the AC is blasting. However, it should be condensation, never refrigerant.

Since the HVAC system is a closed-loop, it means that your car relies on the same supply of refrigerant to keep everything cool. When refrigerant leaks, there’s no replacement for the fluid, so you don’t have enough refrigerant to do its job.

4. HVAC Air Temperature Never Changes

I should mention that the air doesn’t necessarily have to be hot in order for there to be a problem. If your AC is stuck blowing cold air, then it could also be due to a failing AC compressor. The compressor is responsible for turning on and off the AC, so if it’s not turning off that’s also an issue.

Close up of an HVAC climate control inside a vehicle with max AC air conditioning on

If the compressor is stuck on, then it will wear out even quicker and break altogether.

The other culprit is the dial that selects the temperature. I had a car where the dial was broken, and I thought something was wrong with the HVAC system. It’s going to be easier for you to start with the dial and then move to the compressor after.

5. Corrosion on the Compressor

If you take a look at the compressor, look for corrosion. You can do this by popping the hood and grabbing a flashlight, you don’t need to remove the compressor first.

Corrosion anywhere is a bad sign. The rust could also be inside your compressor, causing the bearings to fail and destroying the internal fittings.

Close up of a car engine with visible corrosion and rust

If you didn’t know, corrosion is a result of oxygen and moisture coming together. It could also be a result of acid coming in contact with a device, namely from your car battery (though that’s very uncommon in this example). When a moving part has rust on it, the parts will wear out much quicker and the whole unit could seize.

If you find corrosion on your compressor, you should take a closer look at all your other components under the hood. It’s rare for just one piece to corrode.

6. The Compressor Clutch Is Jammed

The clutch in your compressor will toggle the piece on and off, as I mentioned earlier. The clutch is a part that physically moves, so it can get jammed over time and stop working.

The clutch could be damaged, corroded, or not lubricated enough which can cause it to get stuck in one position.

If you look at the front of the compressor assembly, you’ll find a belt that wraps around a toothed piece. The belt is called the serpentine belt, and it connects multiple components together. The toothed piece is part of the clutch on your compressor.

AC-compressor-clutch-highlighted

When it’s engaged, the tooth will be in contact with the serpentine belt, and it will be spinning around the clutch. This is the only thing that drives the compressor, so it’s important that you see motion whenever the compressor is turned on.

If the clutch is jammed or failing, you won’t hear a click when you turn on and off your AC. You can test this by running your car and popping the hood. Stand in front of the car and look at the compressor’s clutch.

Have someone else sit inside the car to toggle the AC button on and off, then turn on the heater, and then press the AC button or knob on again. While this happens, a functional clutch will dance around, engaging and disengaging with the serpentine belt.

7. Electrical Damage

There is also an electrical aspect to your compressor. You’ll find a wire that runs along the top face of the compressor and plugs into the rear side of the unit.

If this wire has any damage to it, then your compressor might not work. It’s worth looking at the connection plug and ensuring the compressor is fully plugged in and engaged. It’s not common for a compressor to randomly unplug over time, but it’s worth a shot to check it.

Close up of an automotive electrical wire that was chewed up and damaged destroyed by rodents

The more common issue would be damage to the actual wire. If you recently go through a mouse infestation in your car, they could have chewed through the wires and broken your compressor. Since the mouse probably didn’t have insurance, it’s now your problem to fix.

Another thing to look for is black stains around the wire and at the connection points. If there was any type of small electrical fire or surge, the leftover soot will let you know. Again, this is very uncommon but worth knowing.

Can You Replace a Bad AC Compressor?

When it comes to AC compressors, rebuilding them is so expensive that it rarely makes sense to do so (unless you have an exotic car and replacement parts are hard to find). Instead, you should just replace your bad AC compressor.

Car mechanic DIY - fixing a car

At your local auto shop or online, it will probably cost a few hundred dollars for a new compressor. As far as the actual labor, you can get it done in a few hours — assuming nothing goes wrong.

The hardest part is usually identifying that the compressor is the problem. Since your HVAC system is made up of a number of parts, it takes some troubleshooting before you can definitively say it’s the compressor. The worst-case scenario is that you change the compressor and the AC still doesn’t work. Even then, you now have a brand-new compressor so it’s not the end of the world, you just need to do more troubleshooting.

How to Replace a Bad AC Compressor in Your Car

In theory, replacing your car’s AC compressor is pretty straightforward. It just entails taking out some bolts, disconnecting the compressor from the serpentine, and taking it out. In practice, however, it can get a little tricky.

Depending on your car, you might have some obstructions that block you from accessing the mounting bolts on your compressor. In this section, I’ll give you a quick guide to replacing your AC compressor after finding out that it’s bad.

Step 1: Locate the Compressor

It all starts with locating the compressor. It looks like a little motor with a black pulley on the end. You’ll probably find it near the front of the engine bay, but it could be near the bottom, under other parts.

Close up of the car's AC air conditioning compressor with belt and pulley visible

A quick way to spot it is to look for your serpentine belt and trace the line.

Step 2: Drain the Refrigerant

Remember, your compressor is moving refrigerant through your HVAC system. That means that you’ll find a line going into the compressor and away from the compressor. Both of these lines are part of the closed-loop of refrigerant that circulates through the engine bay.

If you remove the compressor, these lines will detach and you’ll spray refrigerant all over yourself, the ground, and the engine bay. To avoid this messy situation, you should drain the refrigerant before touching anything.

Mechanic either recharging or draining the car's AC air conditioning system

If you don’t have a recovery system at your house, then don’t even bother with this step. Instead, have a mechanic handle the project for you. Trying to drain and refill refrigerant without a recovery system is very difficult to do perfectly.

If you do have a recovery system, connect the low-pressure line to the “add inlet” of your system. This will be towards the back of the engine bay near the top.

Step 3: Remove the Belt

The serpentine belt is going to get in the way if you try to remove the compressor right now. Instead, you need to remove it.

There’s a tensioner somewhere along the belt that you need to loosen. You can spot it because it has a hinged arm that moves up and down, tensioning and releasing the belt. Loosen the tensioner and the arm will swing, allowing you to get slack in the serpentine belt and remove it.

Car belt pulley tensioner - alternator

Before going too far, take a close look at this serpentine belt. If it looks especially worn, cracked, or damaged, then you should also pick up a replacement belt. Now is a good time to replace it anyway, since you’ll be working on the compressor.

When the belt is removed, be careful where you set it down. Never put it on the ground or on a dirty surface, because the added dirt can get lodged in the belt and wear out components in your engine bay, prematurely killing things like your alternator and compressor.

Step 4: Remove Any Parts That are in the Way

Take a second to look around your compressor. More specifically, look at the mounting bolts on the compressor and determine if you have adequate space to fit a wrench in there and remove them. In addition, see if there’s enough clearance to remove the compressor, or if anything is blocking it.

Person removing the plastic engine cover from a car

There’s a good chance that you can’t right now. The compressor could be buried towards the bottom of your engine bay with three components in the way.

Before removing the compressor officially, you need to uninstall and remove everything in the way. In my recent project, I had to remove my power steering pump before I could remove the compressor. Again, this is going to vary depending on your car, and you might have great access to the compressor without removing anything.

Step 5: Remove the Compressor and Reverse the Steps

Now you can finally remove the compressor. First, you should remove the two refrigerant lines at the top of the compressor. There will be one bolt holding either line in, so remove the bolt and the lines will come off smoothly. Add a little wad of paper to seal the lines and make sure debris doesn’t get in.

Mechanic using a socket wrench to service a car engine wrenching

Then, remove the mounting bolts on the compressor and take it out.

At this point, you’ll want to go back through these steps and undo everything. With your new compressor, install the mounting bolts, install the refrigerant lines, reinstall any parts that you previously removed, put the serpentine belt on the clutch, tighten the tensioner, and add refrigerant.

You’re officially done replacing your compressor.

Conclusion

Now you know everything you need to know about your car’s AC compressor. I just covered what it is, how it works, and 7 easy ways to find out that your car’s AC compressor is bad. If you have other car questions, explore the rest of my page or leave a comment below. I regularly upload new content, so keep checking in for helpful guides in the future. Until then, check out what car products I highly recommend and pick some up for yourself.

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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.

1 thought on “How to Tell if a Car AC Compressor Is Bad (7 Signs)”

  1. When I was driving my way to work this morning, I noticed that my AC was only blowing hot air from its vents, so I was wondering what could be wrong with it. I’m glad that you informed us how hot air usually indicates either a failing compressor or low levels of refrigerant. I’ll be sure to contact an auto service as soon as possible to perform repairs on my car’s air conditioning.

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