Enhanced Evaporative System
The OBD II, Enhanced Evaporative system, runs self-monitoring tests under certain conditions. A leak caused by a cracked or loose gas cap will set a fault code and illuminate the check engine light to notify the driver. The owner may think something more serious has occurred. If a scan indicates an enhanced evaporative leak, check the refueling cap for looseness.
The most common leak codes are P0440 and P0442. P0440 indicates a large leak above 0.060 inches, and P0442 indicates a minor leak as small as 0.020 inches. The system code typically indicates a leak but may also indicate a blockage in the system. These systems are pressure-sensitive. On newer vehicles, the ECM self-monitors and applies vacuum to the tank and then monitors it for a certain period for leakage.
This system prevents leaking fumes that pollute the environment with HC's (hydrocarbons are unburned fuel). In addition, today's vehicles contain an onboard refueling vapor recovery system. It keeps the fumes that are pushed out of the tank while refueling in the system.