P0446

DTC P0446 Evaporative Emission (EVAP) Control System Vent Control Circuit Malfunction




Circuit Description
The evaporative system includes the following components:
- Fuel tank.
- EVAP vent solenoid.
- Fuel tank pressure sensor.
- Fuel pipes and hoses.
- Fuel vapor lines.
- Fuel cap.
- EVAP canister.
- Purge lines. EVAP canister purge valve.
- EVAP service port.

The evaporative emission system is checked by applying vacuum to the EVAP system and monitoring for a vacuum decay. The PCM monitors the vacuum level through the fuel tank pressure sensor signal.

At an appropriate time, the EVAP canister purge valve and the EVAP vent solenoid are turned ON, allowing the engine to draw a small vacuum on the entire evaporative emission system. After the desired vacuum level has been achieved, the EVAP canister purge valve is turned OFF, sealing the system.

A restricted or blocked EVAP canister vent path is detected by drawing a vacuum on the EVAP system, turning OFF the EVAP vent solenoid and the EVAP canister purge valve (EVAP vent solenoid Open, EVAP purge PWM 0 %) and monitoring the fuel tank vacuum sensor input. With the EVAP vent solenoid open, any vacuum in the system should decrease quickly unless the vent path is blocked. A blockage can be caused by the following conditions:
- Faulty EVAP vent solenoid (stuck closed).
- Plugged, kinked or pinched vent hose.
- Shorted EVAP vent solenoid driver circuit.
- Plugged evaporative canister.

If any of these conditions are present, Diagnostic Trouble Code P0446 will set. DTC P0446 is a type B code.

Conditions for Setting the DTC
- The BARO is greater than 72.3 kPa.
- No: MAP, TPS, IAT, ECT, EGR, EVAP, VSS or System Voltage Diagnostic Trouble Codes.
- IAT and ECT at start-up are between 3.5 °C (38 °F) and 32 °C (90 °F).
- The difference between IAT and ECT at start-up is less than 6.75 °C (12.2 °F).
- The Fuel Tank Level Sensor reads between 10 % and 90 %.
- The difference between the actual Fuel Tank Pressure and the expected Fuel Tank Pressure is less than the PCM's expectation.

- Fuel Tank Pressure slope is less than or equal to 1 mm (0.03 inches) of water. The above conditions are met, and the following condition is met once:

Fuel Tank vacuum is greater than 39 cm (15.35 inches) of water.

Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The PCM will records operating conditions at the time the diagnostic fails. The Freeze Frame and Failure Records buffers will store this information.
- A history Diagnostic Trouble Code is stored.

Conditions for Clearing the MIL/DTC
- The MIL will turn off after 3 consecutive ignition cycles in which the diagnostic runs without a fault.
- A history DTC will clear after 40 consecutive warm up cycles without a fault.
- A Tech 2 can clear the DTC's.

Diagnostic Aids
Check for the following conditions:
- Poor connection at PCM: Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
- The Fuel Pressure Sensor shares a 5 Volt reference with the MAP sensor and TP sensor.

If these codes are also set, it could indicate a problem with the 5 Volt reference circuit or components itself.

- The Fuel Pressure Sensor share a ground with the MAP sensor and the TP sensor.
- Damaged harness: Inspect the wiring harness to the EVAP vent solenoid and the fuel tank pressure sensor for shorts to ground, shorts to battery positive, and open circuits.
- Kinked, pinched or plugged vent hose: Verify that the vent hose between the canister and the EVAP vent solenoid is not restricted.

Check for charcoal particles. Refer to EVAP System before starting repairs.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that causes the Diagnostic Trouble Code to be set occurs. This may assist in diagnosing the condition.

Steps 1 - 4:

Steps 5 - 8:

Steps 9 - 12:

Steps 13:

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Chart:
1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the Tech 2 if applicable. This creates an electronic copy of the data taken when the malfunction occurs. This may assist in diagnosing the condition.
3. If a vent solenoid electrical malfunction is present, the purge system will not operate correctly. Repairing the electrical malfunction will very likely correct the condition that set Diagnostic Trouble Code P0446.
4. Checks the fuel tank pressure sensor at ambient pressure.
6. Forces the fuel tank pressure sensor to re-zero.
7. Verifies that the fuel tank pressure sensor accurately reacts to EVAP system pressure changes.
9. Checks for a blocked EVAP canister.
13. Duplicates the Powertrain On-Board Diagnostic test.