What is the horrific smell from the air conditioner?
It smells nasty, like mildew. Could it be a plugged drain hose?
In my experience with a terrible smell coming from the A/C vent, there was a leaking heater hose going into the car that caused the problem.
Being that I live in Hawaii, the heater hose was cut and then capped. No more foul odor!
There are some common situations that cause bad smells to come from A/C ducts.
— Leafy build up often in the intake, but also along the duct’s run. Leaves can rot, decay, and create a smell.
— Drain hose clog. This causes water to back up and collect in the lower areas of the duct work where mildew can develop.
— Small rodents. It has been reported that mice or similar creatures crawl into the A/C system. A/C systems are similar to the underground passageways that rodents naturally build and prefer. They may die or, at least, leave droppings in the duct system.
Removal & Elimination:
Easiest will be to clear a clogged drain hose. Check for this first and take appropriate action. Sometimes an additional few inch length of hose added to the end of the car’s drain hose and routed properly will prevent another occurrence.
If it is not a clogged drain hose, attempt to isolate the specific duct or ducts that are worse. This may provide a direction to pursue.
Disassemble the duct work in the worst area searching for any problem listed above.
If a build up of leafy material or animal droppings is found, clean the duct work thoroughly leaving no residue. Use various cleaners until no residue remains.
Be certain to thoroughly clean the junctures between the duct’s different sections.
Once clean and odor-free, discover how the problem originated and take steps to eliminate and prevent a recurrence.