Tighten a pipe wrench around the nut at the center of the sewer drain cap. Turn the nut counterclockwise to loosen the cap from the threads at the top of the drain.
Spray the edges of the drain cap with a penetrating catalyst if you could not loosen it. A penetrating catalyst is a product that helps loosen threaded items that are stuck. Spray a liberal amount of the catalyst around the threads of the cap and let it soak for 10 minutes.
Tap the sides of the nut in the center of the cap gently with a hammer to help loosen it. Use the pipe wrench again and attempt to turn the nut counterclockwise. Apply more penetrating catalyst and tap the nut with the hammer. Eventually you will be able to loosen the cap and remove it by hand.