Air conditioning almost never fails in the winter. It fails on the first genuinely hot week of the year, because that is the first time you have asked it to do real work. A system that is slightly low on refrigerant will still take the edge off a mild day and then give up completely at 28 degrees.
The three things that usually cause it
1. Refrigerant has slowly leaked away
Every air conditioning system loses a small amount of refrigerant a year through its seals and hoses. That is normal, not a fault. Lose enough of it and the pressure switch shuts the compressor down to protect it, so the fan still blows but nothing cools.
2. The condenser is blocked
The condenser sits at the front of the car, ahead of the radiator, and collects flies, leaves and road grit. Once airflow through it is restricted the system cannot shed heat, and it is at its worst when you are sitting in traffic with no natural airflow.
3. An electrical fault, not a refrigerant fault
Pressure sensors, the compressor clutch relay and the cabin controls can all stop the compressor engaging while the gas charge is perfectly healthy. Topping up refrigerant on a car with an electrical fault wastes your money and tells you nothing.
What you can check before booking
- Set the temperature to its coldest and the fan to maximum, then listen for the compressor engaging — a small change in engine note.
- Check the recirculation button is on. Pulling in 30-degree outside air makes any system look weak.
- Look at the front grille for a mat of flies or leaves.
- Replace the pollen filter if it has been more than two years. A clogged filter reads as weak air conditioning.
- Note whether it is warm all the time or only at idle. Only at idle points at airflow or the condenser fan.
Why a regas on its own is often the wrong answer
If a system has lost its charge, something let it out. A regas without a leak test buys you one more summer at best. A proper job puts the system under vacuum, holds it, and finds where it drops before any refrigerant goes in.
Newer cars use R1234yf rather than R134a, and the two are not interchangeable. Any garage working on your car should be able to tell you which one it takes before it starts.
When to stop investigating and get it looked at
Get it booked in if the air is warm at every speed, if you can hear a rattle or squeal when you switch the air conditioning on, if there is an oily film on the pipework under the bonnet, or if it has been regassed within the last year and is already weak again. That last one is a leak, not a top-up.