Circuit Description
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 DTC P0446 diagnostic,
the EVAP purge solenoid is commanded to a fixed duty cycle (pulses valve) and the
EVAP vent solenoid is commanded On (closed) in order to build vacuum. Once vacuum
is obtained, the EVAP vent solenoid is commanded Off and the vacuum should decrease
rapidly. DTC P0446 sets when the vacuum decay is too slow indicating a blocked
vent system.
Conditions for Setting the DTC
DTC P0446 will set if fuel tank pressure sensor voltage is less than
0.70 volts during the P0446 diagnostic test indicating a blocked vent system
when:
| • | Condition exists for longer than 1 minute and 40 seconds |
| • | EVAP vent solenoid is commanded Off (open) |
| • | EVAP purge solenoid is commanded to PWM (open) |
| • | Fuel level is between 15-85% |
| • | Baro is greater than 75 kPa |
| • | ECT and IAT at engine startup are between 4°C-30°C (39°F-86°F) |
| • | ECT and IAT are within 8°C (46°F) of each other |
| • | TP angle is between 7 and 35% |
| • | Vehicle speed is less than 113 km/h (70 mph) |
| • | No ECT, fuel tank pressure, IAT, MAP, O2S-1, PCM internal fault, TP or
VSS DTCs have been set |
DTC P0446 diagnostic is run once (usually for 30 seconds if no
faults exist) during the EVAP diagnostic test once the above conditions have been
met.
P0446 is a (type A) DTC
Diagnostic Aids
Important: If DTC P1640 is set, diagnose that DTC first. A short to ground causing
the EVAP vent solenoid On, will cause the vent valve to close. This will cause DTC P0446
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.
Check the following for causes of a blocked vent system:
| • | EVAP vent solenoid blocked or stuck valve |
| • | Vent hose plugged, kinked or pinched |
| • | EVAP canister restricted |
DTC P0446
