car air conditioning: why it stopped being cold

air conditioning does not wear out gradually in the way brakes do. it usually works, and then does not, and the cause is nearly always one of a small number of things.

it is almost always refrigerant, and that means a leak

the commonest fault by a distance is low refrigerant. and here is the part that matters: the system is sealed, so refrigerant does not get used up. if it is low, it leaked.

which is why a top-up on its own is a temporary fix. put gas in without finding the leak and you will be doing it again next summer, having vented refrigerant into the atmosphere in between.

proper diagnosis means finding the leak, usually with a dye or an electronic detector, and fixing it before recharging.

the other causes

a failed compressor, the expensive one. often preceded by a squeal or a grinding noise, and sometimes caused by having run the system low on refrigerant for a long time, since the refrigerant carries the compressor's oil.

a blocked condenser, the radiator-like part at the front. leaves, bugs and road debris block airflow through it and the system cannot shed heat.

a blocked cabin air filter. the air is cold, there is just not enough of it reaching you. cheap, and often overlooked.

electrical faults, a pressure switch or a relay, which stop the compressor engaging at all.

why running it in winter is worth doing

run the air conditioning for ten minutes every few weeks through the winter. the compressor's seals are lubricated by oil carried in the refrigerant, and a system that sits unused for six months is a system whose seals dry out and start leaking.

it is also how your defroster works. the air conditioning removes moisture from the air, which is why the windscreen clears faster with it on.

what we do

find the leak before adding gas. a recharge with no diagnosis is not a repair, it is a delay you paid for.