DIY: Moderate
A/C Blowing Warm Air: Causes and What to Check First
A/C that blows warm has a short list of common causes: not enough refrigerant, a compressor that is not turning on, a cooling fan that is not running, or a blend door problem behind the dash. What you notice narrows it down.
Stay safe first
- A/C trouble alone is not a reason to stop driving. If the temperature gauge climbs or you hear a loud grinding from the compressor, turn the A/C off and have it checked.
- Refrigerant can cause frostbite on skin and should not be vented. If you are not experienced, let a shop handle anything that involves opening the system.
Air that is only slightly cool and has been getting worse
Likely causes
- Low refrigerant from a slow leak
- A leaking seal, hose, condenser or compressor
What to check
- Check whether the compressor clutch engages. With the engine running and A/C on, the center of the compressor pulley should spin with the belt, with an audible click when it starts.
- A recharge can may bring cooling back for a while, but if refrigerant was low there is a leak, and it will come back.
- Ask a shop to find the leak, using dye or an electronic leak detector, before adding more refrigerant.
Warm air and the compressor never clicks on
Likely causes
- Blown A/C fuse or a failed relay
- Refrigerant so low that a pressure switch keeps the compressor off
- Failed compressor clutch or compressor
What to check
- Check the A/C fuse and relay in your owner's manual. They are the cheapest things to rule out.
- If the fuse and relay are fine, a low-pressure cutout from very low refrigerant is the next likely cause.
- If the clutch tries to engage but the system still does not cool, have the compressor checked.
Cold while driving but warm at stoplights
Likely causes
- Condenser cooling fan not turning on
- Condenser blocked with debris or damaged fins
- Refrigerant overcharged by a do-it-yourself recharge
What to check
- At idle with the A/C on, look at whether the fan in front of the radiator is running. If it is not, the fan or its relay is the likely problem.
- Look through the front grille at the condenser for leaves, bugs or bent fins.
- If you added refrigerant yourself, too much can cause this. Ask a shop to check the pressures.
Cold on one side of the car and warm on the other
Likely causes
- Low refrigerant on cars with separate temperature controls
- Failed blend door actuator behind the dash
What to check
- Listen behind the dash for a repeating click or tapping when you change the temperature. That often points to a blend door actuator.
- If it is the actuator, cooling is fine and only the mixing is off, so a refrigerant recharge will not help.
Narrow it down for your vehicle
The same symptom can have different likely causes depending on your year, make, model and engine. Describe what you are seeing and CaliperAI will help narrow down the likely causes for your vehicle.
General information about this symptom, not a diagnosis of your specific vehicle. Safety-critical work such as brakes, fuel and airbags is best left to a qualified mechanic.