Step 3 · Check Air Filter and Indoor Unit

Open the access panel to the indoor air handler (usually located near the furnace or air conditioner). Locate and remove the existing air filter. Examine it for dirt and debris. If it appears clogged, replace it with a new filter of the correct size and MERV rating. Inspect the area around the air handler, including the drain pan, for any signs of water leaks or microbial growth.