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We turned on the air conditioning in our car last week to test it and it smells musty. What causes this and how can it be removed? – Melanie in Montreal

Getting prepared for the summer's heat waves is a wise move.

Experts say air conditioner odour is a common problem, and one that's amplified in humid climates.

"The air conditioning system not only cools, it dehumidifies the air. What causes the odour is bacteria and mould growing in the evaporator core, due to excess humidity and moisture. The more humidity, the worse it is," says Fred Rose of Anglo Canadian Automotive Supply in Vancouver. Rose has been servicing air conditioning systems since 1978.

Particles pulled in with the outside air (bacteria, spores and dirt) typically drain from the vehicle along with condensation formed when the air conditioning is in use. When excess moisture is present, these particles instead settle in the warm, moist evaporator and case. It's dark, and ripe for the growth of bacteria and mould.

To eliminate the smell, you have to get rid of the bacteria. But how?

"There are various chemicals and procedures. We use ozone, and run an ozone machine in the car while the vehicle is running so that the ozone is pulled in through the air intake and kills the bacteria. That procedure takes a couple of hours, and then the car has to sit for some time. Because ozone isn't good for us either, you have to give it time to disperse," says Rose.

There are other products on the market aimed at combatting the bacteria. "One is a mist treatment, using a product which foams up. You drill a hole in the case and then introduce the mist to the evaporator core and let it sit, give it time to work, and then flush it out," says Rose.

Spraying disinfectant in the intake vents on a regular basis isn't likely to solve the problem. "The disinfectant spray odour within the car can get rather strong, and it doesn't really effectively kill the bacteria – but masks it with another smell." The most effective method, according to Rose, is a mist or ozone treatment.

As with many auto makers, Subaru Canada offers tips for preventing air conditioner odour: avoid parking under trees to reduce the possibility of leaves entering the air intake, use the fresh air setting on your climate control rather than the recirculated air setting whenever possible to allow the evaporator to dry out, and drive on paved roads whenever feasible as dust may accelerate the condition.

Whether or not you're experiencing problems, the general consensus is that you should have your air conditioning system checked every two years. This should involve an inspection of the evaporator core and all lines and hoses to make sure no leaks have developed, and cleaning of the condenser.

"You should also take a general look at how it's running, and whether it's cold enough. If the refrigerant charge is down, the compressor starves for lubrication, which can create all kinds of problems," says Rose.

There's also an item in the system called a drier or accumulator. "Its purpose is to remove any moisture, and it has a lifespan of seven years. It should be changed at least that often or every time there's a major repair made to the vehicle, because you don't know how much of the desiccant in that unit is left, and you don't want to take a chance on it bursting because it can create expensive problems," says Rose.

Take your vehicle to the air conditioning professionals to resolve the smell, and any other pending problems.