Car air conditioning fails in two distinct ways, and telling them apart is most of the diagnosis. Either plenty of air is coming through the vents but it is not cold, or the air is cold enough but there is hardly any of it. People describe both as “the AC is not working”, and they lead in opposite directions.
Put your hand at the vents, set the fan to maximum, and decide which one you have.
Quick checks worth doing first
Genuinely common, genuinely easy to miss:
- Is recirculation on? Drawing already-cooled cabin air back through the system cools far better than pulling in hot outside air. On a hot day, recirculation should be on.
- Is the temperature dial actually on cold, and on dual-zone systems, is the passenger side set too?
- Is the AC button on? On many cars, switching to windscreen demist mode turns the compressor on automatically and changes what you are hearing.
- Are the windows shut? Obvious, and worth confirming.
- Have you given it time? A car parked in direct sun starts far above ambient temperature. Driving with the windows down for the first minute to flush the hot air out makes a real difference.
Plenty of air, but it is not cold
This points at the refrigeration side.
- Low refrigerant from a leak. The most common cause by far. The system is sealed, so if the refrigerant is low it has escaped, and simply refilling it without finding the leak means paying for the same job again. This needs a workshop with the right equipment.
- The compressor is not engaging. With the engine running and AC on, the compressor at the front of the engine should click in and its centre pulley start turning. If nothing engages, it may be the compressor, its clutch, a pressure switch protecting the system because the refrigerant is low, or an electrical fault.
- A blocked condenser. The condenser sits in front of the radiator, and it takes the full force of road debris. Leaves, insects and dirt packed into the fins stop it shedding heat. It is visible through the front grille and can often be rinsed gently, with water directed straight through the fins rather than at an angle that bends them.
- The engine cooling fan is not running. It draws air through the condenser at low speed. If cooling is fine on the move and poor in traffic, suspect the fan.
- A stuck blend door. Cars mix cold and hot air to reach the set temperature. If the actuator that controls that flap fails, the system can keep blending heat in regardless of the setting. This often announces itself with a repeated clicking from behind the dashboard.
Cold enough, but hardly any air
This points at the airflow side.
- A clogged cabin air filter. The single most common cause of weak airflow, and on many cars an easy job behind the glovebox that takes minutes. Your manual shows where it is and how to release it. In dusty conditions it needs changing more often than the standard interval suggests.
- Blocked intake at the base of the windscreen, where leaves collect.
- A failing blower motor, which often works only on some fan speeds, or its resistor, which is the usual reason the fan works on maximum but nothing else.
- A frozen evaporator, which can happen when the system is low on refrigerant or the drain is blocked. Airflow drops away after twenty minutes or so and returns after the system is switched off for a while.
Other symptoms worth naming
- A damp or musty smell usually means moisture sitting in the system, often from a blocked condensate drain. Water dripping under the car after running the AC is normal; water in the footwell is not.
- Cooling that fades on long drives and returns after a stop can indicate a system struggling with low refrigerant or airflow.
- Cooling only when moving points at the condenser or the cooling fan.
- A change in engine note when the AC engages is normal — the compressor is a real load on the engine.
About DIY recharge cans
Refrigerant top-up kits are sold widely and are tempting. The difficulty is that an accurate charge is measured by weight, not by a gauge on a can, and the amount involved is small enough that guessing goes wrong easily. Overcharging raises pressures and can damage the compressor, which is the most expensive part of the system. If the system is low, it has a leak, and finding it is the actual repair.
Your manual has the final word
Cabin filter location and interval, refrigerant type and charge weight, and the controls on your particular climate system are all car-specific. Where this guide and your owner’s manual disagree, follow the manual.
