The engine P21AB Code is found on the engine code reader when the engine is tested. When the engine code reader cannot read the area of the engine, then code appears and it is necessary to get the real meaning of the code. The manufacturer of your car put a specific meaning for the car engine and it is necessary to get that meaning for solving the car engine. Do not use the automobile dictionary meaning for the car engine for solving the problem. The automobile meaning of the code is not important to use. You should rely on the real meaning of the code.