If your car’s AC only works while driving, the issue stems from underlying AC problems such as poor air flow, refrigerant problems, or failing components.
Common Causes:
- Faulty condenser fan: Doesn’t pull enough air at idle, leading to poor cooling.
- Clogged condenser: Dirt, leaves, or debris block air flow and refrigerant flow, affecting AC performance.
- Low refrigerant level: Reduces the system’s ability to absorb and release heat.
- Compressor issues: A worn clutch or failing compressor limits refrigerant circulation at a low speed.
- Bad expansion valve: Restricts refrigerant flow.
- Electrical issues: Wiring harness and relay problems prevent the compressor or fan clutch from working at idle.
NOTE: A clogged radiator or faulty radiator fan can cause heat buildup, reducing your car’s AC performance.
How to Fix It:
- Check the condenser fan: Inspect the wiring and fan motor; replace if faulty.
- Clean or replace the condenser: Remove blockages or replace the unit if it’s damaged.
- Test the refrigerant level: Check for low refrigerant level or leaks; recharge or repair leaks as needed.
- Inspect the expansion valve: Look for clogs, sticking, or wear; replace if faulty.
- Assess the wiring: Examine all AC wiring for corrosion or loose connections; repair or replace if necessary.
- Inspect the compressor clutch: Check the clutch plate for wear; replace if worn.
Not sure what’s going on with your AC?
If you need more clarity, this article breaks down why your AC system cools only when driving, the repair options, and costs.
And if you’d rather skip the guesswork, the experts at AutoNation Mobile Service are available to handle your car’s cooling issues right from your driveway.
This Article Contains:
- Why Does Your Car’s AC Only Work When Driving?
- How Do You Fix an AC that Only Cools When Driving?
- How Much Does it Cost to Repair a Faulty Car AC?
- 3 FAQs About a Car’s AC
Why Does Your Car’s AC Only Work When Driving?
When your car AC blows cold air only while driving, it might indicate:
1. Bad AC Condenser Fan
The condenser fan keeps the refrigerant cool by pulling air through the condenser coil. If the cooling fan isn’t working (due to a faulty fan motor), the refrigerant doesn’t cool properly at idle. This causes hot air to blow from the AC vents.
Once you start driving at highway speed, natural airflow takes over, so the AC seems to generate cool air again.
2. Clogged Condenser
The condenser releases heat from the refrigerant. Over time, debris can block the condenser, restricting the flow of refrigerant and reducing the system’s effectiveness. This may lead to poor cooling.
3. Low Refrigerant Levels
The refrigerant absorbs heat inside the vehicle and releases it outside through the condenser.
When the refrigerant is low, the system can’t circulate enough to remove heat efficiently. At idle, the compressor turns slowly and airflow across the condenser is limited, so the AC feels weak. At higher speeds, both compressor activity and airflow increase, making the AC feel cooler on the road, even though performance is still below optimal.
4. Refrigerant Leaks
When the AC system develops a leak, refrigerant slowly escapes. This lowers the refrigerant levels and reduces the pressure inside the system.
If the pressure gets too low, the refrigerant can’t effectively absorb and release heat, and the system struggles to produce cool air. While driving, airflow and compressor speed mask the problem for a while, making it seem like the AC only works while driving.
5. Faulty Compressor Clutch
When the clutch fails (due to a damaged coil), the compressor can’t build enough pressure when idle, causing hot air to flow from the vents. However, once the vehicle is in motion, the high speed boosts the failing clutch, allowing the compressor to push refrigerant through.
6. Bad Expansion Valve
A bad expansion valve restricts refrigerant flow, causing warm air to blow from the vents at idle. While driving, higher pressure forces refrigerant through, temporarily restoring cooling.
7. Bad AC Compressor
The compressor is responsible for pressurizing the refrigerant and keeping it moving between the condenser, expansion valve, and evaporator. If the compressor is wearing out, it may not build up enough pressure when the engine is running at a low speed.
8. Wiring Issues and Faulty Relays
Damaged wires or failing relays can prevent the condenser fan, compressor clutch, or blower motor from working properly at idle. When the car is moving, vibrations or intermittent connections can sometimes restore power just enough to make the AC feel cooler.
NOTE: A failing radiator, faulty electric fan, or worn radiator fan clutch can also affect your car’s AC performance. If these parts don’t pull enough air, heat builds up under the hood, making it harder for the AC condenser to cool the refrigerant.
Now that we understand the possible causes, let’s move on to the repairs.
How Do You Fix an AC that Only Cools When Driving?
It’s best to consult a repair professional as they have the tools and experience to diagnose and fix AC problems correctly.
But if you still wish to know, here’s how a mechanic will go about it:
- Test the condenser fan by running the engine with the AC on full blast. If the cooling fan doesn’t start, they will inspect the fuses, relays, sensors, and wiring with a multimeter.
- Examine the condenser for signs of clogging that could restrict air flow. They’ll clean or replace it as needed.
- Run a vacuum test to check the air conditioning system for refrigerant leaks. Monitor the vacuum reading for five to ten minutes to make sure it holds steady. They’ll inspect refrigerant levels and hoses for low charge or damage, then recharge or repair the system as needed.
- Check the expansion valve to see if it is stuck or clogged. If it’s faulty, the mechanic will replace it.
- Inspect the wiring harness of the AC system components for loose contacts or corrosion. They’ll replace the wiring and connectors if necessary.
- Assess the clutch and clutch plate for wear or damage, and replace them if they are faulty.
- Perform a road test to confirm the AC delivers steady cooling both when idling and driving at higher speeds.
NOTE: If the AC components seem fine but the cabin remains hot, the mechanic may inspect the radiator, electric fan, or radiator fan clutch for issues.
Worried this might prove to be a costly repair?
Keep reading to know more about the expenses.
How Much Does it Cost to Repair a Faulty Car AC?
The repair costs depend on which air conditioning system component needs replacing. Here are some estimates (including parts and labor):
- AC refrigerant hose leak repair: $20 to $35
- AC condenser fan motor replacement: $100 to $700
- AC expansion valve replacement: $200 to $500
- AC refrigerant leak repair: $350 to $2,600
- AC clutch replacement: $590 to $650
- AC condenser replacement: $630 to $780
Still got doubts?
The next section might have your answer.
3 FAQs About a Car’s AC
Here are some commonly asked questions concerning the air conditioner:
1. How Does Your Car’s AC System Work?
A car’s air conditioning works by moving heat from inside the car to the outside, keeping the cabin cool.
Here’s the step-by-step process:
- The AC cycle starts with the compressor pressurizing the refrigerant and turning it from a low-pressure gas into a high-pressure gas.
- The hot, high-pressure gas flows into the condenser, where outside air passes over the condenser coil. Now heat is released to the outside air causing the refrigerant to turn from a gas into a high-pressure liquid.
- Next, the refrigerant moves through the expansion valve or orifice tube, which lowers its pressure before it enters the evaporator.
- Inside the evaporator coil (or the evaporator core), the refrigerant evaporates back into a gas, absorbing heat and moisture from the cabin air in the process.
- The blower fan pushes this cold air through the vents into the cabin. Meanwhile, the absorbed moisture drains out of the vehicle.
2. What Are Some Ways to Prolong Your AC’s Life?
Here are a few simple practices you can follow:
- Replace your cabin air filter every 15,000 to 30,000 miles. You can also check the cabin air filter replacement interval in your user’s manual.
- Make AC servicing a part of your annual car maintenance to identify potential issues.
- Maintain a clean cabin and engine bay to prevent mold buildup in the vents.
- Avoid running the AC immediately after starting the car, and open windows first to let warm air escape.
3. Does Using the Car’s AC Affect Gas Consumption?
Yes, running your car’s AC decreases fuel mileage. It can reduce fuel efficiency by about three to ten percent.
The AC compressor draws power from the engine to operate, causing it to burn more fuel to meet the increased energy demands. This also increases emissions, affecting the environment.
Keeping the air conditioner off and using open windows works well at city speeds. But at high speed, open windows create aerodynamic drag, reducing fuel efficiency.
Enjoy Reliable AC Fixes Without the Repair Shop Hassle
If your car’s AC cools only while driving, it’s a sign your system might need attention. And the longer you wait, the bigger the bill could get.
That’s where AutoNation Mobile Service steps in. We’ll send our expert technicians right to your location, seven days a week.
What’s more?
Every repair comes with upfront pricing and a 12-month, 12,000-mile warranty.
Book your service today for hassle-free car maintenance and repairs.