OBD II Enhanced Evaporative Emissions
Today's vehicles have an enhanced EVAP system, and older vehicles have a basic EVAP system. The EVAP system prevents an excess vacuum and vapor buildup in a sealed fuel tank system. Enhanced systems also monitor operation. The vehicle runs a vacuum or a pressure test to inform the PCM of any leak greater than 0.020 in. (0.5 mm). Certain criteria (the drive cycle) must be met for the test to run. The system runs a large leak test before the small leak test.
A P0457 gross leak detected code is often the result of the vehicle's operator not tightening the refueling cap correctly after refueling. The same is true for the P0455 large leak fault code. A loose gas cap is not the only possible cause, but it's undoubtedly the most likely. A cracked or loose hose, faulty gas tank seams, can cause a significant leak in the sealed tank system.
The PCM runs a large leak test when it receives a signal from the fuel level sensor indicating an increase in fuel level (after refueling). If the fuel filler cap is left loose, the PCM sets a code and illuminates the MIL in an attempt to warn the driver of the situation.