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 the 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). 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. If a correct amount of vacuum is achieved, the PCM will run the
DTC P0442 diagnostic, which will monitor the decay in vacuum of the sealed
system over a calibrated amount of time. DTC P0442 sets when the vacuum decay
is greater than the calibrated amount for a certain length of time due to a small
leak.
Conditions for Setting the DTC
DTC P0442 will set if the vacuum decay is greater than 0.02-0.10 fuel
tank pressure sensor volts per second, 0.02 with low fuel level and 0.1 with
high fuel level, indicating a small EVAP leak when:
| • | The condition exists for longer than 15 seconds. |
| • | The EVAP vent solenoid is commanded ON (closed). |
| • | The EVAP purge solenoid is commanded OFF (closed). |
| • | The fuel level is between 15-85 percent. |
| • | The barometric pressure is greater than 75 kPa. |
| • | The ECT and IAT at engine startup are between 4-30°C (39-86°F). |
| • | The ECT and IAT are within 8°C (46°F) of each other. |
| • | The TP angle is between 7-35 percent. |
| • | The vehicle speed is less than 113 km/h (70 mph). |
| • | No ECT, fuel tank pressure, IAT, MAP O2S-1, PCM internal fault, TP, or
vehicle speed DTCs have been set. |
DTC P0442 diagnostic runs once per ignition cycle, usually for a total of 30 seconds
if no faults exist, once the above conditions have been met.
DTC P0442 diagnostic runs once per ignition cycle once the above conditions
have been met.
DTC P0442 is a type A DTC.
Diagnostic Aids
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 the FUEL TANK PRESS. SENSOR on the scan tool is between
2.35-2.85 volts with the ignition ON and the fuel cap removed.
Use the EVAP Pressure/Purge Diagnostic Chart and Ultrasonic Leak Detector to
locate the small leak.
Possible causes of a small leak:
| • | EVAP vent solenoid valve not seating correctly |
| • | EVAP vent hose loose or damaged |
| • | Fuel sender assembly O-ring leaking |
| • | Fuel tank or filler neck leaking |
The small leak diagnostic can only detect a leak greater than 0.04 of
an inch in diameter.
