OBD II Enhanced Evaporative Emissions
The EVAP system prevents excess vacuum and vapor buildup in a sealed fuel tank system. An enhanced EVAP system is implemented on today's vehicles and a basic EVAP found on older model vehicles. A big difference is the ability of the enhanced system to monitor it's operation. The vehicle will run a vacuum or a pressure test depending on the model and 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. A large leak test is run 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. Please note that a loose gas cap is not the only possible cause, but it's certainly the most likely. It could also be caused by a cracked or loose hose, faulty gas tank seams or anything else that can cause a large leak in the sealed tank system. A thorough visual inspection will often reveal loose hoses and lines.
The PCM will run 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 were left loose, the PCM would set a code and illuminate the MIL in an attempt to warn the driver of the situation.