The on-board diagnostics, or OBD, code P0442 can often be fixed by tightening the gas cap or replacing the gas cap. The code indicates a small leak in the evaporative emissions, or EVAP, system, which is commonly caused by a loose gas cap or a small hole in the cap. Not all EVAP codes can be fixed by replacing the gas cap as there could be leaks in other parts of the car.
Further troubleshooting solutions include performing an EVAP test or using advanced diagnostic tools. These advanced solutions should typically be done by a professional mechanic.
The OBD system, known as OBD-II in most cars, runs on a vehicle's engine control unit (ECU). It continually runs diagnostic tests to detect malfunctions in the engine. If a problem is found, it stores a code that gives an indication of the issue, which can be read with an OBD-II scanner. While an OBD-II code can have several causes, it still narrows down the problem to a general area, such as the EVAP system or a particular component like a knock sensor.
Pulling the codes off an ECU is generally the first step of a diagnostic process. Once a repair is made, the codes can be reset. The check engine light will return if a new code is found because the problem was not completely fixed.