Why does my AC still smell like old laundry even after a cleaning?

That smell is almost certainly **dirty sock syndrome**, wich happens when mold and mildew build up on the evaporator coils and in the drain pan. Cleaning the filters is just surface-level maintenance; it won't touch the source of the stench if it's alredy rooted in the fins. Those dark, fuzzy spots your girlfriend saw are mold colonies thriving on the moisture that naturally accumulates while the unit runs.

Since you're in an apartment, the maintenance guy probably did the bare minimum because pulling the whole unit apart is time-consuming. That blue plastic piece you see could be a shipping bracket that was never removed, or it might be a part of the housing that snapped off. Either way, if it's obstructing the airflow, it's causing more condensation to sit in the pan instead of draining out, which basically creates a petri dish for bacteria. You can actually fix this yourself for about ten dollars without needing a technician. Go to a hardware store and grab a can of **no-rinse foaming coil cleaner**. You'll need to turn off the power, pop the front cover off, and spray that foam directly onto the metal cooling fins behind the filters. The foam will expand, eat thru the grime, and then liquify and carry the gunk down into the drain line as the AC runs. Make sure to wear a mask while you're doing it because those fumes are harsh and you don't want to breathe in whatever spores you're dislodging. If you can reach that blue object with a pair of needle-nose pliers, pull it out. Once the coils are clear and the obstruction is gone, the smell should vanish within an hour of running the unit. Just keep an eye on it afterward to ensure the drain line isn't backed up, or you'll be right back where you started in a month.

While the previous poster correctly identified the biological origin of the odor, I suspect that 'blue plastic piece' is actualy a misplaced condensate tablet or an improperly seated drain pan baffle that has migrated from its intended position, wich typically results in localized pooling of stagnant water. You really ought to verify if the secondary drain port is obstructed by debris because even a slight blockage will significantly elevate the relative humidity within the blower housing, thereby facilitating the rapid colonization of those exact mold spores your girlfriend observed. If you find the no-rinse foam insufficient, you might need to inspect the blower wheel itself since the centrifugal force often flings moisture against the insulation, creating a hidden reservoir for microbial growth that standard coil cleaning simply cannot reach.

Oh wow, I just joined because I'm finally getting my first place and this thread is terrifying but so cool! I had no idea you coud just buy a foam spray and fix it yourself for ten bucks. That sounds like magic! Since you guys are talking about weird things stuck in the vents, has anyone ever found a bird's nest in one? My new apartment has these massive exterior vents and I'm super worried about critters getting in there before I even move my couch in!

Quick update—you guys were spot on about 'dirty sock syndrome.' I grabbed some of that foam spray mentioned in the replies and it worked like magic on the coils. The blue thing was actually a tablet meant for the drain pan that had just shifted. everythign is finally clean and the smell is totally gone. Thanks for the advice!