Evap Code: Loose Gas Cap
A cracked or loose gas cap on an enhanced evaporative system will set a fault code. The system runs self-monitoring tests when certain conditions are met. If the pressure sensor indicates a leak to the PCM, the system sets a code. Some vehicles have a dedicated instrument panel light that indicates a loose cap. This is because customers were leaving their gas cap loose after refueling their vehicle.
The most common leak codes are P0440 and P0442. P0440 indicates a large leak above 0.060 inches, and P0442 indicates a smaller leak, as small as 0.020 inches. The system code typically indicates a problem like a leak, but may also indicate a blockage in the system. These systems are pressure sensitive. The PCM self-monitors and applies vacuum to the system, and then monitors it for a certain period looking for leaks.
This system prevents leaking fumes that pollute the environment with HC's (hydrocarbons are unburned fuel). Today's vehicles contain an onboard refueling vapor recovery system. It keeps the fumes that are pushed out of the tank while refueling contained in the system.