Odour is a normal feature of automobile air conditioners. But, the odor’s pungency can be reduced greatly by identifying the working on the source. An undesirable odor coming from the dash air vents is in most cases caused by mice or other small insects living in the air box. To eliminate this, removal of the nest from the inside of the duct or blower motor area is recommended.it is possible too that this can be caused by mold and or bacteria growing on or around the air conditioner’s evaporator unit or core.
Follow this steps

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