The evaporative emission (EVAP) system is used to store fuel vapors as to reduce the amount of unburned fuel from escaping into the atmosphere. The EVAP system consists of the EVAP canister, fuel tank pressure sensor, EVAP lines and hoses, EVAP purge solenoid (normally closed), EVAP service port, EVAP vent solenoid (normally open), fuel tank, and PCM. The PCM monitors the EVAP system for circuit faults in the fuel tank pressure sensor, EVAP purge solenoid and EVAP vent solenoid circuits. The PCM also monitors the EVAP system for small and large leaks. During the DTC P0440 diagnostic, the PCM monitors the fuel tank pressure sensor for an atmospheric pressure reading when all of the DTC parameters have been met with engine running. The PCM will then command the EVAP vent solenoid On (close valve) and command the EVAP purge solenoid to a fixed duty cycle (pulses valve) with engine running. The PCM will let the pressure drop in the fuel tank for a certain length of time after which it will turn Off the EVAP purge solenoid. DTC P0440 sets when a certain vacuum in the fuel tank could not be achieved due to a large leak.

DTC P0440 will set if fuel tank pressure voltage is greater than 1.0 volts during the P0440 diagnostic test indicating a vacuum cannot be pulled on the EVAP system when:

DTC P0440 diagnostic runs once per ignition cycle (usually for 30 seconds when no faults exist) once the above conditions have been met. If this test fails, a warm test will be performed and can only PASS a diagnostic test, not fail it. The purpose of this test is to keep the MIL Off during the initial test if the customer starts the vehicle with the fuel cap off.

DTC P0440 diagnostic runs once per ignition cycle once the above conditions have been met.

P0440 is a (type A) DTC

Important: If DTC P1640 is set, diagnose that DTC first. A diagnosed EVAP purge solenoid or EVAP vent solenoid circuit fault may have caused this DTC to set.

Important: The PCM uses the fuel tank pressure sensor to detect the amount of vacuum pulled on the EVAP system during the leak diagnostic tests. Make sure the fuel pressure sensor is not skewed by verifying FUEL TANK PRESS. SENSOR on Scan tool is between 2.35 and 2.85 volts with ignition On and fuel cap removed.

Make a visual inspection for a large leak of the EVAP system. Use the EVAP Pressure/Purge Diagnostic Chart and Ultrasonic Leak Detector to locate the large leak.

Possible causes of a large leak:

Important: A very large leak such as a missing fuel cap may be difficult to pick up with the Ultrasonic Leak Detector.