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This code refers to a problem with the evaporation system on your truck. GM has issued a technical bulletin on it for repairs. I will post a copy of it for you. Thanks
#02-06-04-037D: DTC P0446 (Restricted/Blocked Evap Vent Path) Set, Check Engine Light On (Replace Evaporative Emission (EVAP) Vent Valve Solenoid Assembly and Add/Relocate Filter Box Using Service Kit) - (Apr 16, 2008)
This bulletin is being revised to add new style pickups to the models and update the correction information to include a new location for the filter box. Please discard Corporate Bulletin Number 02-06-04-037C (Section 06 -- Engine/Propulsion System).
Some customers whose vehicles are operated in dusty type environments, such as farming or mining off-road type applications, may comment about the illumination of the check engine light, with a DTC of P0446 (Restricted/Blocked EVAP Vent Path) being set.
Dirt and dust intrusion into the EVAP canister vent hose assembly, resulting in restricted air flow, may cause this condition.
After following the published SI diagnostics for P0446 and determining that the EVAP canister vent valve is the cause of the MIL light, replace the existing EVAP canister vent valve assembly with a new assembly. This new assembly is a sealed unit that is designed to be vented through a remote filter box. To ensure correct installation, follow the procedures below.
Important: The new canister vent valve will be installed in a new location, outside of the frame.
Tighten Tighten the bracket mounting bolt to 12 N·m (106 lb in).
After following the published SI diagnostics for P0446 and determining that the EVAP canister vent valve is the cause of the MIL light, replace the EVAP canister vent valve assembly and relocate the remote filter box. To ensure correct installation, follow the procedures below.
19152345 (1999-2003 Model Year)
Solenoid Kit, EVAP Emission Canister Vent Valve
19152349 (2004-2008 Model Year)
Cut to Length
$10.80 USD ($22.59 CAD)
12337820 (10 per pack)
$3.80 USD ($4.55 CAD)
15620999 (10 per pack)
$3.78 USD ($7.08 CAD) (1999-2003)
$7.56 USD ($14.17 CAD) (2004-2008)