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Refrigerant escaping from the condenser or the hoses around it. The condenser sits at the very front of the vehicle, ahead of the radiator, where stones reach it — so it is the commonest single leak point in the system.

Low charge means the module refuses to run the compressor, because the refrigerant also carries its oil, so the complaint is air conditioning that has stopped rather than one that cools badly. Hoses leak at their crimped fittings rather than along their length. Find the leak with dye or a detector before recharging — a system is sealed and loses nothing in normal use, so a low charge always means a leak, and topping up without finding it is a repair with a known expiry date.