mmccann

2012 Q5 3.2L 125K started giving off a smell (inside the car, windows up, ac on) for a few seconds after starting the car not too long ago. Originally, it smelled like mustard and only lasted for about 5 seconds. Now, it smells like rotten eggs/sulfur and only lasts about 2-3 seconds. It seems to be coming from the ac vents, but it vanishes too quickly to really pinpoint the source. No CEL light or other symptoms. I've parked it & have been driving a friend's car, but need to return it to them soon.

(Last issue was cranking/not starting, but that's better since I replaced this a few months ago.) Any suggestions for this new issue are welcome. Thanks

Httq5

If they haven't been changed recently, I would consider changing the cabin air filter with a charcoal filter and also the engine air filter.

MANN - Cabin Air Filter - CUK2450
https://www.ecstuning.com/b-mann-par...k0819439a~man/

MANN - Engine Air Filter - C16-114-2X

stash64

You might need to run a can of Lysol thru your HVAC system. If you live in a humid climate, could be some mold/mildew in the system.

mmccann

I ordered the cabin (charcoal) filter right before posting this! I'll grab an engine filter too. Thank you

mmccann

Yep, 1000% humidity where I live, but hardly any rain for the past 2 years, just dreadful heat & moisture off the bay. Any reason it would decide to do this now and not the past 10 years? Could I do anything to prevent it from happening again?

Httq5

Is it usually parked inside a garage? Is there signs of mold/mildew in the garage? If so, make sure the garage is dry and well-ventillated.

If your drain line is damaged or you frequently use your AC system, the unit is likely to stay damp all the time, which creates a breeding ground for mold and bacteria. As vehicles get older bacteria can start to grow. If the AC unit is not often used, or used much too often, this can also increase the growth of mold/mildew inside the AC system.

Along with changing the filters, you could try turning off the AC 2-5 minutes before you shut off the vehicle but leave the fan on. This will allow the entire AC unit to dry much more thoroughly every time you use it.

Also, I would check the battery. If the battery suffers any damage, (overheating) the sulfuric acid can leak out, causing that rotten egg odor. Hydrogen sulfide is the main reason that makes a battery smell like rotten eggs.

stash64

This is exactly what I do to try to prevent mildew build-up. It would also help to crank up the fan for those last few minutes after turning off the A/C.

The battery suggestion is a good one too. The OP might also want to check to make sure the vent tube(s) are still properly connected to the battery.

mmccann

Parked outside, unfortunately & AC is run for most of the year bc we don't have much of a winter anymore in our area. I will replace both filters, follow your suggestion of running the fan & check the battery. Any advice on checking the vent tube stash64 mentioned? Thanks

stash64

I replaced my battery about a year ago and I believe there was a vent tube on each side, near the terminals. It's a plastic tube and only about 1/4" diameter. It's a simple press fit to vent holes in the battery. On my 2014, the battery is located near the very back and just in front of the tire change tools. No need to remove the spare tire as it is aft of the spare. Lift up the hatch cover and then another small plastic lid to access.

Norskie

I would associate a sulfur/rotten egg smell with ineffective catalytic converters.

If you're positive it's in the HVAC then consider any of the HVAC cleaning solutions.