If your car’s air conditioner is blowing hot air, you probably have some questions. What’s wrong with it? How much will it cost to repair? Can I fix it myself?

In some cases, you may need a professional mechanic to diagnose what is wrong with your car’s A/C, a service for which they (rightfully) charge money. Likewise, there are some A/C repairs that the average person should leave to the pros.

However, in many cases you—yes, even you—can figure out what’s wrong with your car’s air conditioner and fix the problem yourself, all for less than the diagnosis alone would cost at most auto repair shops. You don’t even need any fancy tools - all you need is a can of A/C Pro, which can be found at most auto parts stores and major retailers.

We’re not saying it will diagnose every problem, but it will at least narrow down the options. It lets you know whether you need to take it to a mechanic or can simply fix it yourself.

Here’s how to know what’s wrong with your car’s A/C.

  • What you’ll need
  • What to do, step by step
  • When to seek professional help

What you’ll need and how to diagnose

What you’ll need

Make sure you have gloves, safety glasses, and an A/C Pro gauge. The gauge comes as part of the standard A/C Pro all-in-one solution, but you can also buy it separately (and later add a separate can of A/C Pro if needed).

If you do get the all-in-one kit and end up not needing to use A/C Pro, it’s worth noting that some stores will let you return unopened products for a full refund (be sure you hang onto the receipt!).

You can use the gauge by itself to diagnose the system without opening the can, so if you find that A/C Pro is not the solution, returning it might be an option. (You’ll have to check the store’s return policy to be sure.)

How to diagnose

Start the engine and turn the A/C on maximum

Make sure the fan is on its highest setting and the temperature is at its coldest setting.

Look to see if the A/C compressor is running

Pop the hood and locate the compressor. The A/C compressor is the only belt-driven part with a clutch, and the centre part of the pulley doesn’t turn unless the clutch is engaged. So, if you see a pulley that isn’t spinning even though the belt around it is moving, that’s the compressor.

With the A/C on, it’s normal for the compressor clutch to regularly turn itself off and on. To see how it should work, check out this 20-second video.

If the compressor doesn’t turn on at all with the A/C set on max, there could be something wrong with the compressor. But you won’t know for certain until you check the pressure. Whether the compressor is running or not, checking the pressure is the next step.

Check the pressure with the A/C Pro gauge

To do so, you’ll need to find the low-pressure A/C service port. You can also use our port locator tool to find it. Since the hose attached to the A/C Pro gauge will only snap on to the low-pressure port, you’ll know for certain when you get it right. Once secured in place, the gauge will display the pressure.

What to do next

If the compressor is not running

The gauge is designed to measure the pressure when the compressor is running. When it’s not running, the pressure will be higher than if the compressor were on. In other words, if the gauge reads in the “normal” range with the compressor off, then it’s actually low on refrigerant.

Compressors are designed to shut themselves off if the refrigerant level gets too low. So, if the gauge reads in the “low” or even the “regular” range, the compressor might be working as designed and simply need more refrigerant before it will turn on. You can test this by adding a few ounces of A/C Pro, as explained in the can’s instructions. If the compressor then starts running, you know that the problem was low refrigerant, and you can finish adding A/C Pro until your A/C is running cold again.

If the compressor does not turn on after adding some refrigerant, or if the gauge reads in the “high” range, then you know there is actually something wrong with the compressor itself. To fix the problem, you’ll probably need to take it to a professional.

If the compressor is running

As long as the compressor is on (even if it’s only on for a few seconds at a time), you can use the gauge to measure the pressure. Just make sure you’re reading the pressure during the times when the compressor is running.

If the pressure is low

All you need to do is add A/C Pro until the needle on the gauge is within the recommended range, making sure to check the temperature of your interior centre vent frequently. Low refrigerant is the most common problem with A/C systems, which is why A/C Pro is a fast, easy, inexpensive fix. It recharges the refrigerant, adds the correct amount of lubricant, and seals the small rubber leaks that usually cause the refrigerant to be low.

If the pressure is high while the compressor is running

Do NOT add any refrigerant. In this case, the problem may be that you already have too much refrigerant. It could also mean that there is something wrong with the compressor or the expansion valve. Either way, you’ll probably want to visit a mechanic.

If the pressure is in the correct range but the A/C is still blowing hot air

There is something unusual wrong with the system and you’ll need a professional diagnosis.

One final note: A/C Pro fixes leaks, but only small leaks. If it has taken at least two weeks for your air conditioner to go from “cold” to “warm,” then you have a small leak. If your A/C was cold yesterday and today is blowing hot air, then you either have a major leak or some other problem with the system. That would be one of the situations in which you shouldn’t use A/C Pro.