Troubleshooting Evap System / P0446

jsm10 · August 14, 2021

All,
So my 2007 Silverado had a check engine light. P0446/Evap vent circuit. I have done the following:

1.) Tested vent valve solenoid 25 cycles with a 12V battery charger. Functioned correctly.
2.) Tested purge valve solenoid 25 cycles. Functioned correctly.
3.) Removed hose to vent valve. Blew into it. Held thumb over it for 60s. Air whooshed back out.
4.) Gas cap gasket looks good. I put a little silicon oring grease around it just in case.

I replaced the vent valve solenoid thinking it might be failing intermittently. Still getting the P0446. The system does not seem to have any leak, unless it is very minor (or I would have thought the pressure I blew into it would have bled off).

I am guessing it must be the pressure sensor on top of the gas tank. I don't have a scanner to read real time values on that, though. I read that the WIFI ELM 327 OBD (about $20 on Amazon) will work with a phone with app iOBD2 and provide that real time pressure value (but not sure).

Any thoughts? Buy the ELM or just put a new pressure sensor on it and see if that fixes it? It seems I can feel it over the LHS frame rail and I think it just pulls out.

Thanks in advance!

swathdiver · August 14, 2021

The proper way to diagnose this is with a Tech-2 and a gas cap adapter and smoke machine. Since you've already replaced the vent solenoid, you might want to put a new gas cap on (they just don't only leak from gasket) before replacing the pressure sensor.

From the shop manual:

DTC P0446

Evaporative Emissions (EVAP) Vent System Performance

TYPICAL SCAN TOOL DATA

FTP Sensor Signal Voltage Fuel Tank Pressure
1.5 V Atmospheric Pressure/BARO
Less than 1.5 V Positive Pressure
More than 1.5 V Negative Pressure/Vacuum

CIRCUIT/SYSTEM DESCRIPTION

This DTC tests the evaporative emission (EVAP) system for a restricted or blocked EVAP vent path
that would cause excess amounts of vacuum to be developed in the EVAP system. With the purge valve
open and the canister vent valve open, if the EVAP system vacuum goes above a calibrated threshold,
the test will fail.

CONDITIONS FOR RUNNING THE DTC

● DTCs P0106, P0107, P0108, P0116, P0117, P0118, P0120, P0121, P0122, P0123, P0220, P0222, P0223,
P0442, P0443, P0449, P0451, P0452, P0453, P0454, P0464, P0496, P0608, P0609, P0641, P0651, P1516,
P2101, P2119, P2120, P2122, P2123, P2125, P2127, P2128, P2135, P2138 are not set.
● The ignition voltage is between 11-18 volts.
● The barometric pressure (BARO) is more than 74 kPa.
● The fuel level is between 15-85 percent.
● The engine coolant temperature (ECT) is less than 35°C (95°F).
● The intake air temperature (IAT) is between 4-30°C (39-86°F).
● DTC P0446 runs once per cold start when the above conditions are met.

CONDITIONS FOR SETTING THE DTC

● The fuel tank vacuum is greater than 12 inches H2O vacuum for 5 seconds.

● The FTP is less than -2.5 inches H2O or more than +5 inches for 60 seconds after a cold start.

ACTION TAKEN WHEN THE DTC SETS

DTC P0446 is a Type B DTC.

CONDITIONS FOR CLEARING THE MIL/DTC

DTC P0446 is a Type B DTC.

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 condition.
● An EVAP canister filter that is restricted can cause this DTC to set.

● J 41413-200 EVAP Emission System Tester (EEST)
● GE-41415-50 Fuel Tank Cap Adapter

CIRCUIT/SYSTEM VERIFICATION

1. Ignition ON and fuel cap removed, verify that the FTP sensor parameter is between 1.3-1.7
volts.
○ If not within the specified range, go to the FTP sensor section in Circuit/System Testing.
2. Install the fuel cap.
3. 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 11
inches H2O.
○ If the fuel tank vacuum is greater than the specified range, go to the restriction section in
Circuit/System Testing.
4. 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/Failure Records data.

CIRCUIT/SYSTEM TESTING

IMPORTANT: ● Refer to the J 41413-200 operation manual for detailed instructions in Evaporative
Emission System Diagnosis .
● 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 11
inches H2O.

○ If the fuel tank vacuum is greater than the specified value, isolate the restriction by
disconnecting one component at a time while the EVAP canister purge solenoid valve is commanded to
100 percent and the vent valve is open.

FTP Sensor

1. Ignition ON and fuel cap removed, verify that the FTP sensor parameter is between 1.3-1.7
volts.
○ If the FTP sensor parameter is not within the specified range, test the FTP low reference circuit
for an open or high resistance. If the circuit tests normal, replace the FTP sensor.
2. Connect the J 41413-200 nitrogen/smoke hose to the J 41413-311 brass cone adapter. Disconnect
the hose at the fuel cap end of the GE-41415-50 . Connect the J 41413-311 to the disconnected hose
on the GE-41415-50 . Install the GE-41415-50 filler neck end only to the vehicle.
3. Start the engine.
4. Allow the engine to idle.
5. Use the Purge/Seal function to seal the system with a scan tool.

6. Command the EVAP canister purge solenoid valve to 20 percent.
7. Observe the vacuum/pressure gage on the J 41413-200 and the FTP parameter on the scan tool.
8. Verify that the vacuum increases to the abort limit on the scan tool or more than 3.2 volts, and
the value is similar between the scan tool and the vacuum/pressure gage on the J 41413-200 .
○ If the values are not similar, or the voltage is not within the specified range, replace the FTP
sensor.