Most cars lose around 10% of their air con gas every year through the seals and hoses, so after five or six years there simply is not enough left to cool properly.
Air con that blows cool-ish but never cold is one of the most common things booked in every spring, and the fix is usually far cheaper than people fear.
The most likely cause by a mile
Air conditioning is a sealed system, but it is not perfectly sealed. Refrigerant escapes slowly through rubber hose walls and O-ring seals at roughly 10% of the charge per year — that is normal, designed-in loss. After five or six years without a regas, a system that held 500 grams might be down to 250, and below about 60% charge the compressor cannot generate enough pressure difference to cool properly. It will still blow air, and the air will feel slightly cool, but never properly cold. This accounts for the large majority of not-cold complaints Bob sees.
How to tell a gas problem from a fault
Start the engine, set the temperature to minimum and the fan to maximum, press the A/C button and open a window. Listen and look under the bonnet: the compressor clutch on the front of the compressor should click in and the centre section start spinning. If it never engages, the system has either dropped below the low-pressure cut-out (too little gas) or there is an electrical or compressor fault. If it engages but the air is only mildly cool, you are low on gas. If it engages and cuts in and out every few seconds, that is classic low-charge behaviour.
Other causes worth ruling out
A blocked cabin pollen filter chokes airflow so badly that people mistake it for weak cooling — it is the cheapest thing on this list to change and should be done annually anyway. A condenser packed with leaves, flies and road salt in front of the radiator cannot shed heat, so the system works but never gets cold in traffic. A failed condenser fan does the same thing: cold at 60mph, useless at a standstill. A sticking blend flap actuator can mix hot air in regardless of what the dial says — usually accompanied by a clicking noise behind the dash. And on older cars, a seized compressor clutch or a failed pressure switch stops the whole system dead.
What a regas actually involves
A proper regas is not just topping it up. The machine recovers whatever refrigerant is left, weighs it, pulls the system down to a deep vacuum and holds it to prove there is no major leak, then recharges the exact manufacturer-specified weight of gas plus the correct oil and, usually, a UV dye. If the vacuum will not hold, you have a leak that needs finding before any gas goes in — putting gas into a leaking system is throwing money away and, since refrigerant is a potent greenhouse gas, it is also illegal to knowingly vent it.
Real UK prices in 2026
R134a regas (most cars built before roughly 2017): £50 to £90. R1234yf regas (most cars from 2017 onward): £120 to £220 — the gas itself is far more expensive, which is why the price jumps. Leak detection with dye and a UV lamp: £40 to £80, often bundled. Cabin filter: £25 to £60 fitted. Condenser replacement: £220 to £480. Condenser fan: £180 to £400. Compressor replacement: £450 to £950, and if a compressor has failed internally the whole system needs flushing and a new drier, which is why that job is never cheap. Check which gas your car uses before you compare quotes — a £60 quote and a £180 quote may both be fair for different cars.
How to keep it working
Run the air con for ten minutes at least once a fortnight, even in winter. The refrigerant carries the compressor oil around the system, and a compressor that sits unused for six months runs dry at the seals and starts leaking. Using air con in winter is also the fastest way to demist a windscreen, because the system dehumidifies the air. Change the cabin filter every year. And if it is blowing noticeably less cold than last summer, get it done before July — every garage in the country is booked solid the first hot week.
When not to bother regassing
If the system needed gas last year and is flat again this year, you have a real leak, and another regas is money down the drain. Insist on leak detection first. Likewise if the compressor is noisy or the clutch is slipping, gas will not save it. On a car worth under about £1,500 with a failed compressor, most owners sensibly leave it and open the windows — a £700 repair on a £1,200 car rarely makes sense.
How much does a car air con regas cost in the UK? — £50 to £90 for older R134a systems and £120 to £220 for R1234yf systems used on most cars from 2017 onwards. Always ask which gas your car takes before comparing quotes.
How often should air con be regassed? — Every two to three years is sensible for most cars, because the system loses around 10% of its charge a year naturally.
Why is my air con cold when driving but warm in traffic? — That points at heat rejection — a blocked or dirty condenser, or a condenser fan that is not running. At speed there is enough airflow to cope; at a standstill there is not.
Does running air con use more fuel? — Yes, roughly 5 to 8% in town driving because the compressor loads the engine. At motorway speed the difference against driving with the windows open is negligible.