Car A/C Only Blow Cold Air While Driving? Why & What To Do?

Finding out that your car’s air conditioning system is only working and blowing cold when driving can be an absolute nightmare when the heat and humidity of summer strike. It’s easy to forget to test your car’s A/C during the spring months when you don’t really need it, only to feel the sweat running down your back as soon as the mercury rises.

The most common reasons why your car’s A/C only blows cold air while it’s moving but fails to work when stopped or idling often boil down to issues with the condenser fan, a problem with the condenser, or low coolant levels in the system.

To find out why your air conditioning system only works when your car is moving but does not blow cold when stopped or idling, we will have to look at the mechanical faults that can lead to these issues and what, if anything, you can fix on your own.

Let’s dive in and cool down!

How Does a Car’s Air Conditioner Work?

How Does a Car’s Air Conditioner Work?

In modern-day cars, the air conditioning system is powered by a special compressor which is driven by a pulley connected to the engine’s serpentine belt. When it’s sufficiently powered the compressor pushes pressurized refrigerant, such as freon, into the system.

The R134A Freon refrigerant is converted from a gas into a liquid and then back again. At its coldest point in the cycle, the gaseous Freon is inside the evaporator, where outside air is blown over chilling the air. A blower fan then delivers it to the vents inside the cabin of the car.

Since the compressor is only powered by the motion of the engine’s serpentine belt, the entire air conditioning system of the car stops working when the engine is turned off. This means that even though you might be able to power the fan with the electrical system on, you’ll quickly run out of cold air if the engine isn’t running.

6 Reasons Why Your Car’s A/C Only Works While Driving

The top reasons why your car’s A/C only works when running are low refrigerant levels, a problem with the blower fan, or a compressor problem. Though other potential problems could cause an A/C system malfunction.

Here are the six possible reasons why your car’s air conditioner only cools while you drive and how to fix the issue to get the cold air blowing again.

1. Refrigerant Is Running Low Within The System

Low Refrigerant

Low refrigerant levels in your car’s A/C system are a common reason why it only works while the car is running but does not blow cold while idling. Car air conditioning systems rely on a sealed, pressurized system to change the state of the R134A, and even a tiny leak starts to form in the system, such as in an older hose or the connection to the compressor, it gradually affects the performance of the entire system.

This often starts out as the A/C only working while the car is running and driving down the road. As more-and-more refrigerant leaks out of the system, the problem worsens. Once the R134A Freon levels get too low, the car’s A/C fails altogether.

How to Check Your Car’s A/C Refrigerant Level

You can check your car’s refrigerant levels with an automotive A/C pressure gauge. If you can’t find one at a local auto parts store, you can usually find one online for less than $50.

Steps for Checking Your Car’s A/C Refrigerant Levels

Once you have the gauge, you can test the levels with the following steps.

  • Step One: Locate the low-pressure service port and the high-pressure service port on your car’s A/C system. You can usually find the low port on the passenger side near the firewall and the compressor. You can usually find the high-pressure port near the condenser and the evaporator on the firewall. You can also find them in your car’s owner’s manual or repair manual.
  • Step Two: Attach the low-pressure gauge to the low-pressure port and attach the high-pressure gauge to the high-pressure port.
  • Step Three: Start the car and turn the A/C to the coldest setting. If your car has a “Max AC” setting, be sure to choose it and/or activate the recirculating air feature. This
  • Step Four: Let the car run for at least 7 to 10 minutes.
  • Step Five: Check the gauges once the pressures stabilize.

If the reading on the low-pressure port is between 25 PSI to 45 PSI and the reading on the high-pressure port is between 250 PSI and 400 PSI, then you can rule out low refrigerant levels. If both are below, 25 PSI and 250 PSI respectively, then you are likely low on refrigerant.

How to Fix Low Refrigerant Levels

As a short-term fix, you can fix low refrigerant levels in a car’s A/C system by recharging the R134A Freon levels. This only takes 10 to 15 minutes and should restore the cooling power of the car’s A/C system for a few days or even weeks.

However, the leak that caused the original Freon to escape will persist, and in short order, you’ll find yourself once again with a car A/C that only works while running. Eventually, the refrigerant levels will drop too low, and it will stop cooling at all.

Fixing a leak in a car’s A/C system is usually beyond what a home mechanic can safely do. The location and severity of the leak will factor heavily into how much a mechanic charges to fix it and recharge the system with R134A Freon.

The cost to have a mechanic fix an A/C leak can range from $125 to $650.

2. A Bad/Loose Serpentine Belt

A Bad or Loose Serpentine Belt

If the serpentine belt in your car is bad or loose, it could cause the A/C compressor to be underpowered, resulting in the A/C only working when the engine is running at higher revs.

Why? Well, your car’s entire air conditioning system relies on the compressor to drive the pressurized refrigerant through the phase changes needed to cool the air coming from the blower fan. If your belt is worn-out or loose, or the tensioner arm is on the fritz, the compressor won’t receive enough power to function properly and cause your A/C system only to work when the car is moving, leaving you sweating bullets while you’re idling at a stoplight.

So, if your A/C only operates when your car is on the go, don’t sweat it! Check your serpentine belt and tensioner arm, and get them fixed.

How to fix a Bad Serpentine Belt or Tension Arm

It’s relatively easy to replace a bad or worn-out serpentine belt yourself. The cost for a replacement belt ranges from $55 to $150. Though most are less than $100, and you can replace the old belt yourself in less than half an hour.

If the belt tensioner itself is causing the car’s A/C to only work when running, you might be able to realign it yourself. Though most of the time there’s a fault in the tensioner assembly itself, that needs a mechanic to completely replace it.

The cost to have a mechanic replace your belt tensioner ranges from $175 to $250.

3. Malfunctioning Compressor

A Compressor Problem

A fault in the compressor is another one of the main reasons why a car’s A/C only works when running. This can be a leak in the system, which reduces the refrigerant levels, or it can be the compressor itself failing from long-term use. Many times, the compressor’s bearing burns out and damages the compressor case and other critical components.

How to Fix an AC Compressor Problem

Unless it’s a minor leak, a bad air conditioning compressor usually needs to be completely replaced. This is usually beyond what a home mechanic can do. If you’re lucky and your car isn’t too old, you might be able to find a pre-owned AC compressor for your make and model through a scrap parts dealer, which will save you some money on the mechanic’s repair bill.

The cost to have a mechanic replace the AC compressor in your car ranges from $500 to $1,250. With the cost of parts being a third to a half of the total bill. Though this repair typically includes a total AC system tune-up, recharge, and fixing of any other potential leaks.

4. A Bad A/C Condenser Fan

A Bad A/C Condenser Fan

A problem with the A/C condenser fan can reduce system performance to the point where you can only feel the cold air when your car is running. What’s really happening here is that when you’re driving down the road at highway speed the natural airflow through the engine bay from the grill is maintaining the condenser temperature.

This allows the condenser to act normally without the regulating capabilities of its built-in fan. When you slow down or your car is stopped you don’t have enough airflow to let the condenser work normally, and the vents in your cab slowly start to blow lukewarm air again.

How to Fix a Bad Condenser Fan

If you’re lucky, it might just be a loose wire leading to the condenser fan. You resecure the wire and everything works normally. Though this is relatively rare, and if you’re this lucky, you should also run to the store to pick up a scratch-off lottery ticket.

Most of the time when a car’s A/C condenser fan dies it’s the internal bearing. The entire fan needs to be replaced and the condenser needs to be professionally inspected for faults.

The cost to have a mechanic replace a bad AC condenser fan in your car ranges from $250 to $450.

5. A Bad A/C Control Switch

A Bad A/C Control Switch

If your car’s air conditioning system only blows cold air when it is running, it could be an early sign of a faulty A/C control switch.

The A/C control switch is what powers and controls the A/C compressor, but when there’s a problem with the switch, it can prevent the compressor not to come on to meet the demands input from the interior controls, resulting in no cold air being blown out when the car is stopped.

This is more likely to be an issue for cars with a thermostat-regulated climate control system.

In a scenario like this, the control switch usually also fails intermittently when you’re using the heating system. In some models, the car’s ECU might also throw a code P0531 for “A/C Refrigerant Pressure Sensor Circuit Performance.

The cost of having a mechanic replace your car’s A/C control switch ranges from $120 to $150. Though the part only makes up about $25 to $40 of the total cost.

6. A Clogged A/C Condenser Coil

A Clogged AC Condenser Coil

Debris that clogs the car’s A/C condenser coil can affect cooling performance to the point that the car’s air conditioning system only seems to work when running. Though the real problem here is how the debris got into the sealed system in the first place.

Most of the time, it’s actually the car’s A/C compressor failing that causes tiny flecks of metal and airborne debris to enter the system, where they get caught in the narrow confines of the A/C condenser coil. When dirt and debris get in there, it can significantly impede the system’s proper functioning and can be a major factor in causing your car’s AC not to cool effectively when you stop.

How to Fix a Clogged A/C Condenser Coil

A clogged A/C condenser coil that causes the car’s AC to only work when running typically needs a complete overhaul of the entire system. This usually calls for replacing the condenser coil itself as well as the damaged A/C compressor that likely introduced the debris in the first place.

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

Frequently Asked Questions

How Long Will a Freon Recharge Last?

Recharging your car’s A/C freon levels is at best a short-term fix when your AC stops working while stopped or idling. Just how long the recharged freon lasts will vary depending on the severity of the leak and how much you use the A/C.

I had a minor leak in late August once and a recharge got me through the final three hot weeks of summer weather. I’ve had a major leak where I recharged the freon. I ran the A/C on an all-day drive and the car’s AC was completely dead the next day.

Can I Use R-12 Freon to Recharge My Car’s A/C?

The majority of cars on the North American domestic market made after 1995 use R134A Freon. If your car was made before 1995 your AC system likely uses R-12. The mineral oil in R-12 doesn’t mix properly with R134A or other types of Freon. So, you do need to recharge the system for a short-term fix, make sure you’re using the type of Freon that’s compatible with your car’s A/C system.

Can a Bad Radiator Thermostat Affect A/C Performance

Most of the time the heat of the engine bay doesn’t affect the performance of the car’s air conditioning system. However, if you have a bad AC condenser fan and the engine’s thermostat is stuck closed, the intense heat inside the engine bay might make it hard for the AC to cool even at highway speeds.

Though in a scenario like this, your engine heat indicator on the dash console will also start to rise and eventually peg out, and you’ll need to pull over.


If your car’s AC only works and blowing cold while it’s running, you should start by inspecting the lines for leaks, and if possible use a pressure tester to see if you are low on refrigerant. If you are low on Freon, you might be able to get by for a few days or weeks at the end of summer by recharging it. Though at some point you’ll need to find and repair the leak in the pressurized A/C system.

Your next step should be to check the serpentine belt and/or the tensioner pulley to make sure they are in good working order. If the serpentine belt is loose for any reason or worn out it might not be able to sufficiently power the car’s A/C compressor at low revs.

While you’re under the hood, take a look at the wiring around the condenser fan, as well as the housing around the AC compressor and the condenser coil. If there’s a loose wire or damage to the housing, then you likely have a more serious problem causing your car’s AC to only work when running.

While you might be able to fix a loose wire to get the condenser fan running again, most of the other potential A/C faults are major repairs that need a mechanic’s help.

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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