Your air conditioner is where warm air goes to die before it's reborn as refreshing cool air to be sent into your home. Unfortunately, on rare occasions, a ducted AC unit can be the final resting place for a rodent. The first sign of this potentially considerable problem is a foul smell, which seems to be getting worse.
The worsening smell emanating from your air conditioner is due to the decomposition of the unfortunate rodent. This foul odor may be escaping from a specific vent in a ducted air conditioner, and you may need to rely on your sense of smell to identify the vent in question. Should you call in a professional for further assistance?
Inside the Duct
An air conditioning contractor may be necessary, but you may want to see if there's anything you can do yourself. Your options are fairly limited by how far into the duct you can reach. Carefully remove the wall-mounted vents and attached air filters (you'll need a screwdriver) and look inside the duct (using a flashlight). Can you see the remains of the rodent? Are you able to retrieve it? If so, cautiously remove the rodent's body. Wear rubber gloves, and you may want to wear a dust mask over your mouth and nose if the smell is especially intense. Place the remains in a sturdy plastic trash bag, which must be tightly tied shut and disposed of in your outside trash can. But what if you can't see the remains of the rodent?
As mentioned, your DIY options are fairly limited. Unless you're able to easily remove the rodent, you'll need professional assistance and possible air conditioning repair. The rodent may have passed away further into the AC unit's ductwork, and specialized equipment will be needed to access it. If the rodent was in fact killed by any of the AC's accessible moving parts (such as its fan or fan belt), these components may have been damaged. If so, they'll need to be inspected and repaired or replaced as necessary. Once the rodent has been located and removed, the ducts will need to be cleaned to eradicate the odor.
Duct cleaning involves accessing the unit's air handler, which is where its primary components (evaporator coil and motor) are located. Your home's interior vents will be sealed, and a powerful vacuum device will be attached to the air handler. Once activated, it will circulate air throughout the AC air handler and all ducts, extracting dust, debris, and the lingering smell of the rodent who passed away.
The removal of a dead rodent (and the associated smell) from an AC can be a bigger task than you might expect, and you may need assistance from a qualified repair technician.