An acid test is a chemical field test performed on a system's refrigerant oil to see if it has been contaminated by moisture or high heat. When moisture enters a system, it can react with the refrigerant and oil to create hydrofluoric or hydrochloric acid, which eats away at the compressor's motor windings. If the test comes back positive, a "suction line filter drier" must be installed to clean the system and prevent a repeat compressor failure. Catching acid early can save a customer from a very expensive repair. Technicians usually perform this test after a compressor burnout to ensure the new compressor won't be immediately ruined by contaminated oil.