P0446 is a "vent control circuit malfunction" so the problem should be located somewhere in the vent valve or its wiring. If it were a leak it would be P0442. The early '04-'05 truck fuel system is different so be sure you use the correct year wiring diagram for reference.

 

I had 446 on my 05 a few months ago. Tested everything on that system and through elimination I concluded it was the Fuel Tank Pressure Sensor. You can test the charcoal box by blowing air through it and as long as it flows back out, it works. You can test the purge and vent values by hooking it to 12v (directly from the car battery) and you should hear a clicking noise (vent open/closing). You can run a vacuum on the evap lines and if you maintain the vacuum there is no leaks. If all this is good it is likely the FTPS. There is no way to test this unit. However to replace it you must drop the fuel tank and you’ll see it on top.

 

I replaced the Fuel Tank Pressure Sensor last weekend and the Light came on again and the code came back. At this point, it must be a leak in the line somewhere or the canister itself! Frustrating.

 

P0446 is not due to a leak. From the GM Service Manual:

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.

• 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.

 

I had a P0446 code, and upon doing a check out the system I found that the vent solenoid was working properly and it was opening and closing as commanded by the system, but the problem was with the charcoal canister, or sometimes called fuel vapour storage canister. It was partially blocked and congested and would not allow the gasoline that was entering the gas tank to displace the air and move it out of the system through the vent solenoid. The vent solenoid was opening and closing properly and had proper electrical power communication, it was just that the charcoal cannister was partially blocked and would not allow a free and unrestricted flow of air. I had just purchased a vehicle and I believe the previous owner had gotten gasoline into the charcoal cannister by overfilling the gas filler neck. Anyways, I’ve posted a video and I hope it will help people out… Here’s the link:

 

If you have a OBD 2 scanner that is able to do a fairly simple evap pressure test, what it will do is command the vent solenoid to close. The test is done when the engine is off and not running. I had my scanner hooked into the port, and I had the door open, and the cord to the scanner outside of the truck door when I push the button and initiated the test. I listened closely, and I could hear the flap of the vent solenoid closing. I Just confirmed that the electrical wiring was proper and that the solenoid was being commanded closed.
I now know that the problem was likely with the charcoal cannister and restricted airflow within the cannister itself.
I removed the cannister and the vent solenoid as a unit out of the vehicle. I used my mouth to blow on the port of the charcoal filter side of the canister, where it leads to the vent solenoid, and I could sense a lack of free airflow… Too much air restriction. This in fact was my problem, A partially clogged charcoal canister. Feel free to check out my video as linked in the post below. I replaced the charcoal cannister with a new one, and reattached the old but operative vent solenoid, and it’s fixed the problem of the gasoline not being able to displace the air out of the system.
I drove down to the gas station to try and fill up, and for the first time, the fuel pump nozzle didn’t click off and I filled a full tank all at once!
The OBD 2 scanner that I used was the Seekone OBD 2 scanner. For the price and it’s features it seems this one is one of the best on the market. I ordered it online and it was delivered to my home.
It does live engine data, and as I said it will do an evap pressure test and comand the solenoid to close.
The only thing that I really wish this scanner could do is give me misfire count data for each cylinder, which it does not do.
I do highly recommend the scanner for the features and functionality and its price.