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 will set a code. Some vehicles have a dedicated instrument panel light that will indicate a loose cap. This is because so many 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 inch and P0442 indicates a smaller leak, as small as 0.020 inch. 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 will self-monitor and apply vacuum to the system, and then monitor it for a period of time looking for leaks.
This system is used because leaking fumes 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.