The EVAP system contains the following components:

The EVAP control system collects the fuel vapors from the fuel tank. When the engine is not running, a canister filled with activated carbon (charcoal) stores the fuel vapors. When the engine is running, the stored fuel vapors are drawn from the canister, then mixed with the intake air flow, and consumed in the normal combustion process.

The PCM uses the EVAP purge valve to control the flow of the fuel vapors from the canister. The PCM supplies the ground to energize the EVAP purge valve (purge ON). The PCM controls the ON and OFF time of the EVAP purge valve by a pulse width modulated (PWM) signal. The PCM determines the EVAP purge valve PWM duty cycle based on certain operating conditions (i.e., mass air flow, engine coolant temperature, manifold absolute pressure, and intake air temperature).

The PCM tests the EVAP system by applying vacuum to a sealed system, then monitoring the rate of vacuum decay. The PCM determines the vacuum decay using the signal from the fuel tank pressure sensor. The PCM tests the EVAP system for the following conditions:

The PCM tests the EVAP system, when the following conditions are met:

At an appropriate time, the PCM turns ON the EVAP purge and EVAP vent valves. This opens the EVAP purge valve and closes the EVAP vent valve. The engine draws a small vacuum on the entire EVAP system. After achieving the desired vacuum level, the PCM turns OFF the EVAP purge valve. The PCM detects a leak by monitoring decrease in the vacuum level over a period of time. A small leak in the system causes DTC P0442 to set. A large leak, EVAP or a malfunctioning purge valve will causes an insufficient vacuum level in the system and will set DTC P0440. A large leak may be the result of the following conditions:

The PCM also tests the EVAP system for a restricted or blocked vent path. First, the PCM turns ON the EVAP purge and EVAP vent valves. As indicated above, this opens the EVAP purge valve and closes the EVAP vent valve. The engine draws a small vacuum on the entire EVAP system. Then, at the appropriate vacuum level, the PCM turns OFF the EVAP purge and EVAP vent valves. This closes the EVAP purge valve and opens the EVAP vent valve. At this time, the PCM monitors the vacuum decay using the signal from the fuel tank pressure sensor. Unless a blocked vent path is present, the PCM should detect a quick decay of the vacuum. DTC P0446 will set if a blocked vent path is present. DTC P0446 may be the result of the following conditions:

To check for a continuous purge condition, the PCM turns OFF the EVAP purge valve and turns ON the EVAP vent valve. This closes the EVAP purge valve and opens the EVAP vent valve. The PCM again monitors the vacuum level in the EVAP system. With the purge valve closed and the vent valve opened, the PCM should detect no vacuum in the EVAP system unless a continuous purge conditions is present. DTC P1441 will set if the PCM detects a continuous purge condition. Any of the following conditions will set DTC P1441: