A sewer cleanout cap is a fitting that screws into the cleanout to keep it closed when it is not in use. When a clog forms in the line, removing the cap allows the access into the line to remove the problem.
Contractors do not always install cleanouts when building homes. If they do install them, they are sometimes difficult to find. Potential locations include near the foundation between the house and the street, just outside of bathrooms or inside the home behind the toilet. Since sewer cleanout caps are rarely removed, they sometimes stick. A cast iron fitting heat from a propane torch helps to loosen the cap. If the home has PVC fittings, a hair dryer provides enough heat to loosen the cap safely.