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477 Posts
Discussion Starter #1
P0446 Evaporative Emission Control System Vent Control Circuit Malfunction

My `03 Sierra turned on the check engine light the other day with this one. The obvious gas cap has been checked. Not it. Any ideas what else could be the issue?

Only thing different lately is last to tanks I've run 89 in it instead of 87.

Side note, I unhooked the postive battery cable for a minute and it didn't turn off. Now I know with Fords, you can disconnect the negative battery cable and touch them to the postive battery post to flush the keep alive memory. Same thing possible on GM's?

Any other thoughts? Thanks in advance.

446 Posts
Check all your evap lines. Make sure that the tube or solenoid or whatever is still fully seated where ever it enters the intake manifold.

In this picture you can see my evap box right behind the windsheild washer fluid fill cap in front of the abs computer. Mines got a couple lines coming off of it; one going to the intake and the other going back to the tank. Yours is probably around the same spot and is similar. Check all those hoses. Fiddle around in there and see if one of them is loose or has an obvious break.

Heres another picture to help you locate and identify what i am describing.

477 Posts
Discussion Starter #3
Thanks for the input. I don't have anything like that on my NBS (2003 - 4.8L). I checked what few vaccum lines I have. They are all in good condition. No kinks, cracks, or otherwise.

I did notice a box underneath just in front of the gas tank. Has about 3 lines going into it. One marked purge. Not sure what that's all about. Guess I'll have to continue researching.

I have an extended warranty, but it's such a pain in the ass to take it in to the dealer if it's something I can fix easily. Plus the stupid $50 deductible.

446 Posts
The box under the truck near the tank, did it have a little like green cap on a T in the line that might have said like 1psi max or something? If so that is your evap shit. Inspect that some more. Give me a few and i will post up the trouble code and diagnostics.

446 Posts
This DTC tests the Evaporative Emission (EVAP) system for a restricted or blocked EVAP vent
path. The control module commands the EVAP canister purge solenoid ON and the EVAP canister
vent solenoid ON. This allows vacuum to be applied to the EVAP system. Once a calibrated vacuum
level has been reached, the control module commands the EVAP canister purge solenoid OFF and
the EVAP canister vent solenoid OFF. The control module monitors the Fuel Tank Pressure (FTP)
sensor for a decrease in vacuum. If the vacuum does not decrease to near zero inches H2O in a
calibrated time, this DTC sets.
Conditions For Running DTC
l DTCs P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0120, P0121, P0122,
P0123, P0125, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0147,
P0151, P0152, P0153, P0154, P0155, P0157, P0158, P0160, P0161, P0167, P0220, P0442,
P0443, P0449, P0452, P0453, P0455, P0502, P0503, P1111, P1112, P1114, P1115, or P1120
are not set.
l The ignition voltage is between 10-18 volts.
l The Barometric (BARO) pressure is more than 75 kPa.
l The fuel level is between 15-85 percent.
l The Engine Coolant Temperature (ECT) is between 39-86°F (4-30°C).
l The Intake Air Temperature (IAT) is between 39-86°F (4-30°C).
l The start up ECT and IAT are within 16°F (9°C) of each other.
Conditions For Setting DTC
l The Fuel Tank Pressure (FTP) is less than -10 in. H2O.
l The condition is present for more than 30 seconds.
Action Taken When DTC Sets
l The control module illuminates the Malfunction Indicator Lamp (MIL) on the second
consecutive ignition cycle that the diagnostic runs and fails.
l 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.
NOTE: To locate components, see COMPONENT LOCATIONS . For circuit
reference, see WIRING DIAGRAMS . For connector terminal
identification, see CONNECTOR IDENTIFICATION . If an intermittent
SYSTEM. If testing requires component removal or
replacement, see appropriate REMOVAL & INSTALLATION article.
Conditions For Clearing MIL/DTC
l The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive
ignition cycles that the diagnostic runs and does not fail.
l A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
l A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or
any other emission related diagnostic.
l 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 vent valve inlet, or a pinched vent hose. A blockage in the vent system will also cause a
poor fuel fill problem. If the condition is intermittent, see INTERMITTENT CONDITIONS under
Test Description
The numbers below refer to the step numbers in the diagnostic procedure.
This test determines if the failure is present or intermittent.
Diagnostic Procedure
Fig. 181: DTC P0446: EVAP Vent Control Circuit (Steps 1-6)
Fig. 182: DTC P0446: EVAP Vent Control Circuit (Steps 7-14)

477 Posts
Discussion Starter #7
That's the piece I was looking at in your photobucket. On the surface, it looked ok. Since I'm short of time right now with work and such, inspection is due, and it's under warranty, I decided to drop it off this morning and take the loaner while they mess with it.

Thanks for the help, though. Much appreciated and informative.
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