Main Causes Of AC Only Works When Driving

AC System

Sitting in a car in a car park on a hot day with no possibility of turning the AC on is undoubtedly one of the most frustrating things. If your AC only works when car is moving, it may indicate that there is an internal problem within the system or other parts of your car. 

Read on and learn about the main causes of the AC working only when moving and what to do about it. 

How Does a Car AC Work?

Most car AC systems are powered by the engine via the drive belt. When your car’s engine is running, it will also run the drive belt. This powers various electrical parts of your car, including the alternator and the AC compressor. 

The compressor transforms the refrigerant into a high-temperature gas with high pressure, which enters the condenser. The condenser then removes heat from the refrigerant and turns it into liquid. Then it pushes the liquid into an expansion valve, also known as an orifice tube. 

In the tube, the refrigerant goes back to its original gas form and drops the temperature. Afterward, it is transferred into a receiver or accumulator, which dries the refrigerant. 

Finally, the refrigerant gets to the evaporator, where a blower fan pulls air from the outside through the evaporator core. As a result, cold air flows beyond the evaporator and through the car’s vents. 

Why Your AC Only Works When Driving: Possible Causes

1. Broken AC Condenser Fan

The condenser fan is the essential component of an AC that basically ensures that the system works properly and stays cool.

The main job of a condenser is to transfer heat from an indoor unit to an outdoor unit. If the fan works well, it will reduce the temperature of the refrigerant as it passes through the condenser. 

The AC condenser fans are typically designed to turn on automatically once the car’s engine reaches operating temperature – that is somewhere between 195°F to 220°F. If the fan doesn’t turn on, the refrigerant will not drop to a suitable temperature by the time it manages to reach the evaporator. 

Now you might be wondering – why does the AC still work when the car is moving? Well, the reason the AC is still on when driving is that the air from outside of your car will still go through the condenser. However, since the fan isn’t working, there’s no air to cool down the condenser when your car isn’t moving. That’s why the AC only blows cold when driving. 

2. Low Refrigerant Charge Level

While the AC system is a closed-loop system, meaning that it’s not supposed to leak the refrigerant, unfortunately, leakages may still occur. And this may lead to problems with the functionality of the system.

The leakages may happen because of cracks in the AC system line, or the fluid may leak from the condenser. As the condensers are normally located at the front of the vehicle, they are prone to damage from road debris. This can happen particularly when the car doesn’t have any guards. In some cases, the seals and tubes of the condenser may simply wear down due to long-term use. However, in most cars, it can take even up to 15 years for such a situation to occur.

In any case, leakages will cause your car’s AC system to lose refrigerant over time. Once the fluid is below the minimum level, the compressor will have issues with pumping the refrigerant in the system, particularly when the car is not moving. However, as the vehicle builds up speed, the pumping continues as normal again. 

3. Blocked AC Condenser

Although it’s unlikely for debris to get into the closed-loop AC system, the blockages may still take place. This often happens due to a worn compressor with its internal parts breaking down.

When the internal components of the compressor break away, the debris will get dispersed in the system, which can ultimately cause blockages in the condenser or compressor. As a result, the system will experience issues with recirculating the refrigerant, leading to AC being warm. 

In addition, the debris residue in the refrigerant can also cause blockages. The residue in the form of a sticky paste will cause circulation problems and disturb heat dissipation. Due to a blocked condenser, the AC system will cycle on and off rapidly before it reaches the ideal operation. 

4. Faulty Compressor Clutch

Any blockages in the AC system will likely cause other parts to wear out much quicker. 

For example, the compressor clutch may not be cycling as a result of a broken wire in the clutch coil. When the compressor clutch doesn’t work properly, the compressor itself won’t manage to pump the refrigerant. In such a situation, you will probably notice that the AC in your car doesn’t work at all, no matter whether you’re idling or driving. 

5. Malfunctioning AC Compressor

Compressor is the control center of the AC system. As it’s normally more than one error causing the issue with the cooling system, it may be difficult to identify the problem with the compressor itself at first. 

If there’s a problem with the cooling coil located in the compressor, you will instantly notice that it is no longer as efficient, especially when you’re idling. Another sign you may spot is the noise that comes from the internal parts of the compressor, however, due to the multiple parts the compressor is consisted of, it’s often difficult to determine which part is faulty. 

6. Overheating Engine

The reason why your car’s AC only works when driving may not always be linked with the AC system. The issue can also come from the engine. If your car’s engine is overheating, the temperature within the engine bay will rise, affecting the AC system that is located in the same place. 

Once the high temperature affects the refrigerant and elements of the AC system, the system will have to remove the excess heat. As the car starts moving, the air will pass through the engine bay, helping both the engine and AC parts cool down. This is why even if your engine is overheating, the AC may still work when the car is moving, thanks to the outside air getting in. 

AC Maintenance Tips

Once the AC in your car is functioning well again, the key thing to ensure the system is working properly is to know how to maintain it. Here are a few tips on how to take care of your AC system:

  • Replace the cabin filter. You will usually have to replace it every 30,000 miles, and it will cost you between $15 and $50. 
  • Clean dust and debris regularly. Use a brush or a wipe to remove the residue. This will prevent it from getting into the system and clogging it.
  • Run the defroster once a week for 5-10 minutes. This will help get rid of excess moisture and unpleasant odors. 
  • Get your car’s AC checked regularly. Service the AC system once a year to ensure it’s functioning well. 


Can I repair AC myself?

You shouldn’t repair the AC yourself. The system is quite difficult to deal with, especially  when issues occur, therefore it’s better to take the car to a professional mechanic. 

Still, you can perform some general maintenance checks on such parts as air filters, thermostats, circuit breakers, as well as do AC cleaning yourself. 

How much does it cost to fix AC in my car?

The AC repair costs vary depending on your system. If you need to replace the entire system, you can expect the cost to be anywhere between $200 and $800. 

What’s more, if your condenser fan is faulty, replacing it will cost you from $300 to $500. Other parts of the AC system have a nominal replacement cost, that is is typically under $300. It’s best to consult prices from different sources before making a decision. 

Can I use an OBD2 scanner to test my AC?

Yes, an OBD2 scanner will read AC or climate control codes and data, so you can use it to test your car’s air cooling system. 

Key Takeaways

There are a few possible reasons why your AC only works when driving. The easiest-to-detect issues are the condenser fan and low refrigerant levels.

If the condenser fan is faulty, you will need to replace it. If you notice the refrigerant levels to be too low, you will need to recharge it at a repair shop. 

There are also other possible problems that you may experience with your AC, such as a blocked system, a faulty compressor, or a condenser. In all cases, you will need to have the parts replaced. 

Another common issue that may cause the AC to not work when idling is an overheating engine. You will need this problem resolved as soon as possible before it turns into an expensive engine rebuild.