If you have a squirrel stuck in your AC system, you're not alone. Squirrels can get into a lot of places that don't appear to have any space to move around in. Once they're there, they're difficult to remove without planning, traps and patience. Squirrels generally won't leave any area where they are nesting. If they have babies, they're less likely to leave willingly or often enough to be trapped.
Home heating and air conditioning systems are often small enough for you to reach into crawl spaces and vents to physically remove squirrels. If you opt to try this before using a trap, wear protective clothing, leather gloves to prevent bites and scratches, and a respirator to prevent inhaling dust, feces or possible parasites. If a mother is nesting with babies, put the babies in a box and remove them to an outside location where she can move them to a safer location or relocate the whole family to a wildlife rehab facility.
If you can't reach the squirrel, rent or buy a squirrel trap. Place it near the opening where the squirrel enters and leaves and wait to catch it. Bait the trap with peanut butter, fruit or nuts. After capturing the squirrel, remove any nest material and scrub the area with soapy ammonia to eliminate chemical trails that will attract other squirrels. Seal any openings that other squirrels could use for access.