What this code means
P0446 may mean the EVAP system is not venting the fuel tank normally, so the fuel tank pressure reading can move outside the expected pattern during the EVAP monitor.
What the vehicle may do
- The vehicle may run normally with the check engine light on.
- A restricted vent path can sometimes contribute to a poor fuel fill complaint.
- The concern may be intermittent if the restriction or inlet blockage comes and goes.
Possible fault areas
- Possible restricted EVAP vent hoses, canister, vent solenoid inlet, or vent filter.
- Possible damaged EVAP vent housing or pinched vent hose.
- Possible fuel tank pressure sensor signal concern that should be checked before continuing the P0446 path.
Diagnostic path
Opening context
On this Sierra, P0446 is an EVAP vent system performance code. In plain terms, the ECM may be seeing the fuel tank pressure move the wrong way because the EVAP system cannot vent normally. The truck may run fine, but it can have an emissions light, and a restricted vent path can also show up as a hard-to-fill fuel tank. Broadly, think possible vent-path restriction, vent solenoid inlet issue, damaged vent housing, pinched vent hose, or a fuel tank pressure reading that does not match what the system is being commanded to do.
Start with the basic checks
Before chasing P0446 directly, start with the basic system checks and follow a structured diagnostic approach. If other EVAP electrical or fuel tank pressure related codes are present, handle what they mean first, because they can change the path for this code. Then turn the ignition ON and verify the related vent and purge control codes are not set before continuing with the P0446 performance checks.
Check the fuel tank pressure sensor at vented pressure
With those preliminary checks out of the way, turn the ignition OFF, remove the fuel tank filler cap, and turn the ignition back ON. The fuel tank pressure signal is centered around 1.5 V at Atmospheric Pressure / BARO. Positive pressure reads less than 1.5 V, and negative pressure or vacuum reads more than 1.5 V. With the tank opened to atmosphere, verify the scan tool Fuel Tank Pressure Sensor voltage parameter is between 1.3–1.7 V. If it is not in that range, pause this P0446 path and diagnose the fuel tank pressure sensor signal concern first. Do not guess at a vent restriction until the pressure signal makes sense. The system also needs to be able to vent pressure or vacuum back near 0 in H2O when the path is open.
Run the purge response check
If the pressure sensor voltage is between 1.3–1.7 V, reinstall the fuel tank filler cap. Let the engine idle at operating temperature for 5 min. Then command the EVAP Purge Solenoid Valve to 50% and watch the Fuel Tank Pressure Sensor parameter. The value should not increase to greater than 2.5 V. If it goes greater than that, move into vent system testing. If it stays at 2.5 V or less, reproduce the operating conditions, or reproduce the captured conditions, and see whether P0446 resets.
Understand what the monitor is looking for
The monitor gates are specific, so do not treat this like a code that runs under every drive condition. For this monitor, the ignition voltage is between 11–32 V, the start up intake air temperature is between 4–34°C (39–93°F), the start up engine coolant temperature is less than 35°C (95°F), the barometric pressure is greater than 70 kPa, and the fuel level is between 10–90%. The test runs once per cold start for no greater than 17 min. The diagnostic formula is looking for fuel tank pressure behavior that indicates the EVAP system is not venting correctly. One stage looks for a pressure sensor reading greater than 2.5 in H2O of pressure or less than –5 in H2O of vacuum for 60 s. Another stage looks for less than –12 in H2O of vacuum for 5 s before 10 L (2.6 gal) of purge volume. After the code sets the first time, 2 L (0.5 gal) of fuel must be consumed before it can set the second time.
Test for restriction in the vent path
Keep verification separate from testing. If the verification points you into testing, check the EVAP vent path for blockage or restriction and repair as necessary. Focus on the vent hoses, the canister, the vent solenoid valve, and the vent filter. An intermittent fault can come from a damaged vent housing, a temporary blockage at the vent solenoid inlet, or a pinched vent hose, so inspect the path carefully instead of only looking for a hard failure.
Set up the sealed system pressure test
After any disconnected EVAP hardware is reconnected, remove the fuel fill cap and connect the CH 41415-30 to the fuel tank filler neck. Then connect the GE 41413-A to the CH 41415-30. Use the scan tool to command the EVAP Purge/Seal function to Not Venting so the EVAP system is sealed. Set the nitrogen/smoke valve on the GE 41413-A to nitrogen.
Check vent release after pressurizing
Use the remote switch to activate the GE 41413-A and pressurize the fuel tank to 5.0 in H20. Do not exceed the specified value, because going higher may give you incorrect test results. Then command the EVAP Vent Solenoid Valve OFF and verify the scan tool Fuel Tank Pressure Sensor parameter is 0 in H20. If it is not 0 in H20, test for a blockage or restriction in the EVAP vent hose or inlet. If the inlet and hose test normal, the supported repair path is to replace the EVAP vent solenoid valve.
Verify the repair
If the Fuel Tank Pressure Sensor parameter is 0 in H20 during that vent command check, reproduce the operating conditions again, or reproduce the captured conditions. If P0446 resets, pause, recheck the earlier diagnostic path, and avoid guessing. If it does not reset, the system is OK at that point. After completing the repair, verify the repair and confirm the code stays gone.
Takeaway
The key with P0446 is to prove the pressure signal first, then prove whether the vent path can actually release pressure when commanded. Keep the verification step separate from the restriction testing, and do not condemn the vent solenoid until the vent hose and inlet have tested normal. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0446 diagnosis often comes down to proving the fuel tank pressure signal, then proving whether the EVAP vent path can release pressure when commanded.
For more guided automotive diagnostics, visit STEP Diagnostics.
