A car air conditioner cleaner or fogger treats the musty smell that develops in a vehicle ventilation system, cleaning through the ducting and evaporator that a spray cannot reach.

That smell comes from bacteria and mould growing on the evaporator, which stays damp because it is the coldest surface in the system and condensation collects on it. A fogger works by releasing an aerosol into the cabin with the air conditioning recirculating, so the treatment is drawn right through the ducts and across the evaporator.

For a lasting result, replace the cabin filter at the same time, since a dirty filter will simply reseed the system. A useful habit is switching the air conditioning off but leaving the fan running for the last few minutes of a journey, which dries the evaporator before the car is parked.