GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC P0446: Evaporative Emissions (EVAP) Vent System Performance

Circuit/System Description

This DTC tests the evaporative emission (EVAP) system for a restricted or blocked EVAP vent path. The engine control module (ECM) commands the EVAP canister purge solenoid valve open while the EVAP canister vent solenoid valve remains open. The test will fail if the vacuum level increases above a calibrated level.

FTP Sensor Signal Voltage

Fuel Tank Pressure

Low, Approximately 1.5 V or less

Positive Pressure

High, Approximately 1.5 V or more

Negative Pressure/Vacuum

Conditions for Running the DTC

    • Before the ECM can report DTC P0446 failed, DTCs P0442 and P0496 must run and pass.
    • DTCs P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0128, P0443, P0449, P0451, P0453, P0454, P1516, P2101, P2108, P2119, P2120, P2125, P2138 are not set.
    • The ignition voltage is between 10-18 volts.
    • The barometric pressure (BARO) is more than 74 kPa.
    • The fuel level is between 15-85 percent.
    • The start-up engine coolant temperature (ECT) is less than 30°C (86°F).
    • The start-up intake air temperature (IAT) is between 4-30°C (39-86°F).
    • The start-up ECT and IAT are within 9°C (16°F) of each other.
    • DTC P0446 runs once per trip when the above conditions have been met.

Conditions for Setting the DTC

    • The FTP sensor is more than 8 inches H2O vacuum for 2 seconds during the 13 minute test.
        OR
        The FTP is less than -2.5 inches H2O or more than +5 inches H2O for 3 seconds after a cold start ignition ON.
    • The fuel tank vacuum is greater than a calibrated amount for a calibrated period of time.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

    • An intermittent condition could be caused by a damaged EVAP vent housing, a temporary blockage at the EVAP canister vent solenoid valve inlet, or a pinched vent hose. A blockage in the vent system will also cause a poor fuel fill problem.
    • An EVAP canister, vent hose, or vent solenoid valve that has restricted flow may cause this DTC to set. Using a purge solenoid command with a scan tool will allow vacuum to be applied to the system instead of pressure. With the engine running, the EVAP canister vent solenoid valve open, and the EVAP canister purge solenoid valve commanded to 100 percent, the fuel tank vacuum should not increase to more than 5 inches H2O.
    • An EVAP canister filter that is restricted can cause this DTC to set. Refer to Evaporative Emission Canister Replacement.
    • Disconnecting one component at a time while the EVAP system is under flow will help to pinpoint a restriction in the system.

Reference Information

Schematic Reference

    •  Evaporative Emissions Hose Routing Diagram
    •  Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

  1. Ignition ON and fuel cap removed, verify that the FTP sensor parameter is between 1.3-1.7 volts.
  2. If not within the specified range, go to the FTP sensor section in Circuit/System Testing.
  3. Install the fuel cap.
  4. With the engine running, the EVAP canister vent solenoid valve open and the EVAP canister purge solenoid valve commanded to 100 percent, the fuel tank vacuum should not increase to more than 12 inches H2O.
  5. If the fuel tank vacuum is greater than the specified range, go to the restriction section in Circuit/System Testing.
  6. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame Records data.

Circuit/System Testing

Important: Perform the Circuit/System Verification before proceeding with the Circuit/System Testing.

Restriction

With the engine running, the EVAP canister vent solenoid valve open, and the EVAP canister purge solenoid valve commanded to 100 percent, the fuel tank vacuum should not increase to more than 12 inches H2O.

FTP Sensor

  1. Ignition OFF, disconnect the harness connector at the FTP sensor.
  2. Ignition OFF, test for less than 5 ohms between the low reference circuit terminal A and ground.
  3. If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the FTP sensor.

Repair Instructions

    •  Fuel Tank Pressure Sensor Replacement
    •  Control Module References for ECM replacement, setup, and programming

Repair Verification

Important: Larger volume fuel tanks and/or those with lower fuel levels may require several minutes for the floating indicator to stabilize.

With the engine running, the EVAP canister vent solenoid valve open and the EVAP canister purge solenoid valve commanded to 100 percent, the fuel tank vacuum should not increase to more than 8 inches H2O.