What's Causing Your Car AC to Stop Cooling When It's Hot Outside and What You Can Do About It

Your car’s air conditioning needs to be at its best when the mercury starts to rise. So, it’s especially frustrating if your AC stops cooling every time it gets hot outside. You’re left sweating through your seat covers wondering what’s wrong and if there’s any way to fix it fast!

A lot of times when the AC underperforms or stops working in the hottest of weather it’s a problem with the condenser, which doesn’t have a quick fix, and can be an expensive repair. Though there’s also a chance there’s a simpler problem afoot like the blower fan or a loose wire.

To keep you from getting too hot under the collar, and perhaps spare you a hefty repair bill, we’ll need to take a closer look at how your car’s air conditioning system works and the symptoms of common AC faults.

How a Car’s Air Conditioning Works

Your car’s air conditioning system works by tapping into tricks of chemistry and physic to change the state of a pressurized refrigerant. This starts with a compressor, powered by a pulley that spins with the serpentine belt. Internally it takes low-pressure refrigerant gas and compresses it into high-temperature, high-pressure gas.

The gas is then moved through lines to the condenser, which is usually mounted in the front of the engine bay just behind the grill. The condenser reduces the temperature of the refrigerant while still maintaining it in a high-pressure state. The refrigerant then transitions from a gaseous state to a liquid state as it cools. Forced air helps transfer and dissipate the heat.

The refrigerant then passes through a dryer to remove humidity. A special expansion valve reduces refrigerant pressure, which converts it back to its gaseous state in the evaporator. The air that then blows across the evaporator is cool and dry.

While all of these different components are important, it’s the condenser that dissipates the most heat energy. The condenser is also the component that is most affected by the outside conditions.

Top 6 Common Reasons Why Your Car AC Stops Cooling on Hot Days

A problem with the condenser is the most likely reason why your car’s AC stops working or performs poorly in hot weather. Though it’s not the only thing you should check. The most likely cause of an air conditioning failure like this include:

  • An Exterior Blockage Around the Condenser
  • An Interior Condenser Blockage
  • A Cooling Fan Problem
  • A Refrigerant Leak/Low Refrigerant Level
  • Loose or Shorted Wires
  • The AC Lines Are Frozen Up

1. An Exterior Blockage Around the Condenser

An Interior Condenser Blockage

If you’re lucky it’s just a bunch of dust, grime, and maybe an unknown mouse nest clogging around the external housing of the condenser, making it hard to dissipate heat in hot weather. You can find the car’s AC condenser up by the grill, behind the radiator. If there’s a bunch of junk or even a lot of caked-on dust, it can make it hard for the condenser to dissipate heat efficiently in the AC cycle.

How to Fix an External AC Condenser Blockage

Some careful cleaning and a blast of compressed or canned air might be all you need to clear the blockage. Just be careful, as this is a very hot part of the car and you could burn your hand if you’re careless.

If possible, let the engine bay cool down for an hour or two before testing the car’s air conditioning system again. This will let the ambient heat around the condenser and the radiator dissipate as much as possible before starting the cycle all over again.

2. An Interior Condenser Blockage

An Interior Condenser Blockage

If you check the car’s AC condenser and it looks reasonably clean, you should worry that there’s an internal blockage causing the air conditioning to fail in hot weather. This is one of the worst-case scenarios, as most of the time when you get an internal blockage in the condenser, it’s caused by a broken piece of the AC compressor!

If it is an interior condenser blockage, you’ll likely notice it making some ugly noises when the car’s air conditioning is running. You might even see an excessive amount of condensation. In the case of a severe clog, you might also spot drips of refrigerant leaking from the condenser or the lines around it.

In the case of a minor blockage, the condenser is underperforming and is thus at its worst in hot weather. Yet, you’d still have a little cold air when you first start the car and on more fair-weather days. If it’s a complete blockage, the car’s AC wouldn’t blow any cold air from the get-go, and you’d likely see a bunch of leaking refrigerant in the line from the compressor to the condenser.

How to Fix a Clogged AC Condenser

Usually, the only want to fix an internal blockage in a clogged AC condenser is to completely replace it. Though the larger concern here is that the blockage could be a damaged piece of the car’s AC compressor. This can easily be a double whammy that a certified mechanic needs to take care of.

The average cost to have a car’s air conditioning condenser coil and compressor replaced ranges from around $1,000 to $1,200. Though at least half of the cost or more is in labor, some models can come with a repair bill of up to $2,000 or more.

3. A Cooling Fan Problem

A Cooling Fan Problem

While you’re checking the condenser, also check the nearby cooling fan for faults, which could affect AC performance. If the blower won’t turn on or is weak it won’t be able to help cool the condenser. This is a problem that will be even worse in hot weather when the system is already strained.

Unfortunately, there are a lot of things that can cause a fault in the cooling fan. This includes electric circuit failures, a burned-out capacitor, corrosion on the connectors, or the fan motor itself being burned out.

In a case like this, the engine and the car’s cooling system will likely also be running hot. You can perform a visual inspection by turning on the car with the AC set to max. If the system is operating correctly the blower fan should come on in a few minutes on a hot day. If it doesn’t or it comes on, but it’s weak, you should test it with a multimeter to confirm that it’s getting power.

How to Fix a Bad Cooling Fan

If you don’t immediately see signs of short-circuited wires or corroded contacts, and the fan is running weak or not at all, then you most likely need to have it completely replaced. This is within the realm of what a modestly competent DIY mechanic can handle. Though there’s no shame in having a mechanic take care of the repair.

The cost to have a mechanic replace a car’s cooling fan usually runs around $150 to $250.

4. A Refrigerant Leak/Low Refrigerant Level

A Refrigerant Leak/Low Refrigerant Level

Car AC systems are prone to leaks in the lines as they age, and low refrigerant can cause the entire air conditioning system to underperform when stressed. It also means that the system isn’t fully pressurized.

This is a problem that’s even more likely to show up in overly hot weather. Not only does the system struggle to keep up with demand, but you’re likely running the air conditioning more, which is allowing an increasing volume of refrigerant to leak out.

Sometimes you can spot these refrigerant leaks with a quick visual inspection of the engine bay and the air conditioning lines. Though the leak can be a tiny crack in the compressor, the evaporator, or the condenser in a place that you can’t easily see.

Though it’s relatively easy to test the refrigerant level to either confirm or rule out a leak as the reason why your AC stops working in hot weather.

How to Fix an AC Refrigerant Leak

If your system is low on refrigerant, it’s relatively easy to recharge it. Though it means to be a completely sealed and pressurized system. So, if your refrigerant is low, it’s definitely leaking out somewhere. This kind of repair usually requires a mechanic to find the leak and replace the defective component.

The cost to fix an AC refrigerant leak can vary from as little as $120 for something like a small crack in the line, to as much as $800 to replace a bad component like a cracked condenser housing.

5. Loose or Shorted Wires

 Loose or Shorted Wires

A wiring problem in the blower motor or the air conditioning system’s relationship with the heater core can also cause the AC to feel weak or stop working in hot weather. This can feel a little bit like chasing electrical gremlins, but it’s worth a shot.

Most of the time, a shorted-out wire is located in the wiring harness behind the glove box or in the dash. For some cars, it’s nearly impossible to access this area without breaking an interior panel or two. If you can get at it, you will usually find the wiring for the blower, the evaporator, and the wiring for the control unit.

How to Fix Loose or Shorted Wires

Check all the wires and the connectors they go into for signs of corrosion. If you can get the wires out of the protective conduit, look for signs of damage to the protective coating.

Any corroded connections will need to be cleaned and reconnected. Any damaged or shorted-out wires will need to be replaced or pig-tailed. These are things that most DIY mechanics can handle. Though if you think it’s out of your depth, or you’re afraid that you’ll damage a dash panel trying to access the wires, there’s no shame in having a mechanic take care of this.

The cost to have a mechanic inspect and repair loose or shorted AC wiring can run from $75 to $250 or more. It mostly depends on the labor time it takes to access the wiring behind the dash.

6. The AC Lines Are Frozen Up

If you’re running your car’s air conditioning at the max during hot weather, the lines can freeze up from overuse, which will cause the entire AC system to stop working. In a case like this, it’s usually the interior portion of the evaporator that is responsible for absorbing and reducing the temperature of the air.

If it’s gotten dusty and/or your cab air filter is clogged, small ice crystals can start to build up, which affects performance. If you access your car’s cabin air filter and pull it out to find dust and ice/frost, you’ll have found the culprit causing the car’s AC to struggle in hot weather.

How to Fix Frozen AC Lines

If it’s just a clogged air filter and high demand that caused the AC lines to freeze up, then cleaning the filter, and vents and removing the moisture will usually get the car’s air conditioning running normally again. This is something an average DIY mechanic can handle.

Though, often when the AC lines freeze up, it’s because there’s a refrigerant leak somewhere in the system, often near the receiver/dryer or evaporator, which requires a mechanic to inspect the system and make repairs.

The cost to inspect the lines and replace components like the AC evaporator can range from $125 to $400.

Frequently Asked Questions

Can a Dirty Interior Air Filter Cause the Car’s AC to Fail?

A dirty interior air filter on its own usually won’t cause the car’s AC system to fail. It usually takes another fault like a leak in the refrigerant or a fault in the evaporator, and the two problems compound each other. Then the problem is at its worst when you push the air conditioning system in hot weather. Still, it’s a good idea to clean your interior air filter every 30,000 miles or as part of your spring maintenance.

Will Refilling My Refrigerant Make My Car’s AC Work Again?

If your car’s AC is low, adding new R-12 refrigerant might get it working again for a little white. However, this is supposed to be a sealed and pressurized system. Meaning the only way you could have low refrigerant is if you have a leak. Any new refrigerant you put in will gradually leak out. How long that takes depends on the leak size and how much you run the car’s air conditioning system.

Conclusion

Air conditioning faults tend to show up the most when it’s hot outside. This is usually due to you pushing the system to the max to keep up with the weather, and the fault finally rears its ugly head from a minor issue to a major problem.

If you’re lucky, it’s just some debris around the condenser. You should be able to spot this and clean it yourself in less than half an hour.

Though a lot of times, it’s a blockage inside the condenser itself that’s causing the car’s air conditioning system to fail in hot weather. Unfortunately, the most likely source of the clog is a damaged, broken-off piece of the AC compressor, and both units need to be replaced by a mechanic.

Refrigerant leaks that leave the system low on coolant can also cause the car’s AC to fail in hot weather. This can also cause the lines to freeze up near the evaporator. While you might be able to clean the system and recharge the lines, the leak will just cause the problem to recur shortly.

Jason Farrell

Written By

Jason Farrell

Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.

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