One unique part of living here in the valley is that our vehicle’s ac system gets used almost year round. You may have noticed that over time the air coming from your vents has a foul odor to it. The main cause of this is the bacteria that build up on the evaporator core due to the moisture in the system. The evaporator is located inside of the dash and looks like a radiator but it has refrigerant passing though it instead of engine coolant. You may not know it but your AC system not only cools the air coming into your vehicle, it also removes moisture. This is why your AC compressor comes on when you are also using the defroster setting. When the air moves across your evaporator core that is being cooled by the refrigerant, the air cools and the moisture is removed. This is why, when it very humid, you see the water dripping under your vehicle when the AC is on.
When you system is turned off the evaporator is still wet from the moisture and bacteria starts to form. Now when the air moves across the evaporator the smell comes with it and into the cabin of your vehicle. Most vehicles are equipped with a cabin air filter. Regular changes of this filter will help lessen the odor but will not eliminate it. The filter is mainly there to filter out dust and debris but a charcoal infused filter will help with odor control. To eliminate the issue strong disinfecting foam needs to be applied to the evaporator to kill the fungi. The foam is injected into the evaporator housing and it penetrates into the fins of the evaporator. The foam kills the bacteria and will leave it smelling fresh. This should be an annual service along with changing the cabin filter.
To help your evaporator stay cleaner longer, use your climate control in fresh air mode and not recirculation. Heavy use of recycled air causes a higher level of bacteria to form. You have either a fresh or recycled setting on the climate control unit or you may have a max AC setting. Use the fresh air or non Max AC option. Turn off the AC unit a few minutes before turning off the car and this will allow the evaporator to dry faster. Keep your cabin filter clean to allow maximize air flow.
As the case is with most maintenance items on your vehicle, the more proactive you are with your preventive maintenance, the less issue you will have with that system.