Short answer: When car aircon is not cold, the usual causes are a refrigerant leak, a compressor or clutch that is not engaging, a cooling fan that is not running, or a blocked condenser. Weak airflow is often a clogged cabin filter. A pressure test shows which one it is, so the fix is not just a gas top-up.
Notice when it is not cold
The pattern tells a mechanic a lot. Cold while driving but warm at traffic lights often points to a cooling fan or a blocked condenser. Cold for a while and then warm can be a system icing up or a compressor that keeps cutting out. Never cold at all usually means very little refrigerant or a compressor that is not engaging. Weak air from the vents, even on the highest fan speed, is a different problem: the air is not getting through.
A refrigerant leak
Car aircon is a sealed system. Refrigerant does not get used up, so if the level is low, it has leaked out somewhere: a hose, a joint, the condenser at the front of the car, or the evaporator behind the dashboard. Topping up the gas without finding the leak only buys time, and the aircon will go warm again.
The compressor or its clutch
The compressor is the pump that moves refrigerant around the system. On many cars you can hear a click under the bonnet when the aircon is switched on, as the compressor clutch engages. No click, a grinding or rattling noise, or a compressor that keeps switching on and off can all mean a compressor, clutch or control problem.
Cooling fans and the condenser
The condenser sits in front of the radiator and gets rid of the heat taken out of the cabin. It needs air flowing through it. At low speed and at idle that air comes from the electric cooling fan. A fan that is not running, or a condenser clogged with dirt and leaves, makes the aircon weak in traffic, which is exactly when you notice it most in Singapore.
Weak airflow and smells
If the air from the vents is cold but there is not much of it, the cabin filter may be clogged, or the blower fan may be failing. A musty smell usually means a dirty filter or mould on the evaporator. These are often simpler fixes than a leak or a compressor.
What you can check yourself
Make sure the temperature is set to cold, recirculation is on when it is very hot outside, and the fan works on every speed. With the engine running and the aircon on, look at the front of the car to see whether a cooling fan is spinning. Note any noises or smells. Don't open the aircon system or add refrigerant cans yourself: overfilling or using the wrong gas can damage the system.
What the workshop checks
A workshop checks the system pressures and refrigerant level, looks for leaks at the hoses, joints, condenser and evaporator, and tests the compressor, its clutch or control valve, and the cooling fans. The cabin filter and airflow are checked too. Once the cause is found, you can be told what needs doing before any work starts.
Common questions
Why is my car aircon cold when driving but warm at traffic lights?
This usually means not enough air is passing through the condenser when the car is stopped. A cooling fan that is not running or a dirty, blocked condenser are the common causes.
Will a gas top-up fix aircon that is not cold?
Only for a while if the gas level is low, because a low level means there is a leak. The leak should be found and repaired so the aircon stays cold.
Why is the air from my vents cold but weak?
Weak airflow is often a clogged cabin filter or a failing blower fan, not a problem with the cooling part of the system.
Is it safe to keep driving with aircon that is not cold?
The car is usually safe to drive, but a noisy compressor should be checked soon, and a cooling fan that is not running can also affect engine temperature. Watch the temperature gauge.
