A foul odor from dash outlets when the A/C is on can be caused by what?

A foul odor from dash outlets when the A/C is on can typically be attributed to both a clogged evaporator drain and bacterial growth on the evaporator.

When the evaporator drain is clogged, moisture can accumulate within the system instead of being effectively drained away. This stagnant water creates an environment conducive to the growth of bacteria and mold, leading to unpleasant odors being blown into the cabin when the A/C is in operation.

Moreover, the evaporator itself, being a damp and cold component, provides an ideal breeding ground for bacteria and other microorganisms. This can also result in foul smells as they grow and multiply, further intensifying the odor when the air conditioning is used.

Selecting both A and B accounts for the dual nature of the problem: the physical blockage that prevents proper drainage and the biological issue of bacterial growth, highlighting the complexity of the system and the factors that can contribute to odor issues in automotive A/C systems.