To detect a small leak on the evaporative emission control system, a mechanic uses a smoke machine and looks for smoke emitting from any EVAP component in the system. Visible smoke indicates the source of the leak. Vehicles generate trouble code P0442 when small leaks occurs, but a fuel cap or tank that is damaged or defective also triggers the code. A smoke test of the entire EVAP system typically reveals hidden leaks behind or under vehicle components.
The P0442 code reports that there is a small leak in the EVAP system, but the code really means that when the powertrain computer runs the evaporative leak test twice, the EVAP system does not hold a specific vacuum level for a specific time period. If a 0.020-inch pinhole exists in the EVAP system, the system typically fails the computerized leak test.
If close examination of the accessible components, hoses and fuel cap do not reveal signs of deterioration or damage, the mechanic may conduct the smoke test. During the test, the mechanic should take special note of the carbon canister, the fuel tank filler neck, fuel tank, fuel level sending unit, and the fuel pump position and seal. Small system leaks can occur when someone replaces a vehicle's fuel pump but does not install it or seal it properly. If the leak is in the fuel tank, the mechanic may need to take the rear seats out to gain access to it.