P0446 Evap System Code

[Another JR]

My 2021 3500 gas truck with 13k miles of non-dusty driving just had the check engine light come on, and the code shown is P0446 for a problem with the evap system.  It shows the code stored 3 times. The only other behavior I’ve noticed that might be relevant is that, a few weeks ago, I noticed that I was no longer hearing the fuel pump run for a couple seconds when I opened the door. Then, several days ago, I noticed the fuel pump was back to running for a couple seconds on opening the door. I’m guessing that might have been three start cycles ago. 
 

Are there any known issues with this system where I should start first or should just do the standard troubleshooting steps?  I already checked both fuel fill flappers and they seem Ok.  Thanks for any advice. 

[Chuck FB]

My first thought is ... I hate all this needless crap !, evap, no normal filler cap that seals out dirt or could be locked etc.

 

Before I bought my truck I was wondering what I could do to keep out dirt, mud, water, snow, as I live on a gravel road and there is no way that filler door keeps everything out from landing on that outer flap. I had seen a comment somewhere talking about using a rubber end cap for plumbing systems so I bought one and slipped it over the neck. The door just nicely closes if the rubber cap is slid down all the way onto the filler neck. They call it a 2" but it measures larger than that to go over the plumbers sized pipe and it just nicely slips onto the filler neck with some resistance. So far in the temps I have use the truck ( hard to say what it will be like in 40 below ) its been fine. When I fuel up I pull it off and set it where the DEF blank is so its right there to plunk back on when I have filled. And yes I took the hose clamp off as its not required.

 

No this is no fix to what your dealing with but might be of use in the future if you find your seeing contaminants sitting on that outer flap. Here's a link to the item.

 

https://www.homedepot.com/p/Fernco-2-in-PVC-DWV-Flexible-Cap-PQC-102/100372303

[Another JR]

Thanks. Right now I don’t think dirt is the issue. The truck has mostly been in my barn or on paved roads, with the little bit of off road driving not in dusty conditions. I’m thinking it’s the valve not responding or it’s the pressure sensor, but I haven’t started troubleshooting.
 

I’m past the bumper to bumper warranty period, and I assume this would not be covered by the powertrain warranty. Otherwise I would let the dealer fix it. 

[Chuck FB]

35 minutes ago, Another JR said:

Thanks. Right now I don’t think dirt is the issue. The truck has mostly been in my barn or on paved roads, with the little bit of off road driving not in dusty conditions. I’m thinking it’s the valve not responding or it’s the pressure sensor, but I haven’t started troubleshooting.
 

I’m past the bumper to bumper warranty period, and I assume this would not be covered by the powertrain warranty. Otherwise I would let the dealer fix it. 

While I would assume that too as per the warranty having run out, it wouldn't hurt to check as I suppose this would fall into the emissions system category. I assume you must have access to a shop manual and a two way diagnostic tool as otherwise one would be lost in how to figure anything out with today's vehicles. I feel so lost with a vehicle like this because of its complexity and no proper information or tools to work with it. Which also goes back to that two alternator setup in how it actually works or the fuse box that has more than one fuse that says trailer power but instead ends up being the permanent mounted fuse on the second battery that you already had knowledge of.

 

But back to your issue, the call is free to the dealership and all they can say is no, and you would know where that puts your options. I know I've been warned to be careful not to "fill" the tank as in cram it full as old vehicles have no issues with that as they have no evap system to worry about. I suppose if one filled the tank a little too much and the fuel in the tank as well as the fuel that filled it was cold, then the vehicle was not driven any distance and sat in the sun, I wonder if that is enough to expand the fuel enough to bring on an issue like this.

Edited April 12, 2025 by Chuck FB

[Another JR]

Thanks. I hadn’t thought in terms of it being emissions equipment. I’ll talk to the dealer. 

[Another JR]

I checked the manual and it looks like it’s covered for 7 years. 

[Chuck FB]

That would be fantastic if it does, for sure bring this up as warranty to the dealer and see what their response is.

 

Which brings up a comment of what I had just booked my truck to have the dealer deal with and that is the steering wheel is off to the left some. I was physically there at the service desk and said the truck was off like this from the day I bought the truck and so I want to book it in for an alignment to sort it out. What I had not said as I thought it was self explanatory given that my truck has few miles on it, that of course he would book it in as a warranty claim. I thought today, I should just check and make sure and this time I called and happened to be talking to the lady that is the service assistant manager and said I was just checking to make sure it was written up as a warranty claim ... nope, it wasn't by the service adviser. I have a feeling he did that on purpose because the way I explained it he knew it was this way from my purchase of the truck. If a dealer can slip out from a warranty and get money in their hands, they will. Slight of hand actions like that do NOT impress me and yet this is what one seems to have to deal with. Anyway she made the change of status on the booked in work order and she said they do cover it on warranty for the first 12000 km.

[lundimpact1775]

P0446 looks like possibly fuel tank pressure sensor. Assuming all else in the evap system functions properly, a malfunctioning ftp sensor seems to often be the culprit per some quick research through these forums. Seems fairly common on multiple generations of Silverado. No idea how to replace on these new models you'd have to do more research on that.

[rjgvt]

I put the rubber cap on my fuel filler after I read about it here. After I go through the car wash, there is always water sitting on the cap. I always wonder what would happen if the cap wasn't there. 

[Another JR]

It turned out to be the evap system purge valve leaking or stuck open. That’s the valve located at the charcoal canister. It was covered under the emissions warranty. 

[Chuck FB]

17 minutes ago, Another JR said:

It turned out to be the evap system purge valve leaking or stuck open. That’s the valve located at the charcoal canister. It was covered under the emissions warranty. 

Its always good to walk out of the dealership with a small victory and the liners of your front pockets in your jeans not turned inside out !

[Chuck FB]

On 4/14/2025 at 5:36 AM, rjgvt said:

I put the rubber cap on my fuel filler after I read about it here. After I go through the car wash, there is always water sitting on the cap. I always wonder what would happen if the cap wasn't there. 

The other day washing my truck and its not that its even been plastered yet since driving it off the lot but there was a bit of dust and crud in the gas tank filler area so because of that plumbers cap firmly on the filler I flipped open the filler access cover and gave it a quick wash with the spray held back so its not intense. After I pulled out of the wash and had it parked I pulled the rubber cover and not a sign of moisture around the filler flap. I am so used to taking fuel caps off of everything to fill that its second nature and its so easy to do with this rubber cap and it sits nice in the pocket area of the filler area when gassing up.

[carkhz316]

On 4/17/2025 at 3:23 PM, Another JR said:

It turned out to be the evap system purge VENT valve shorted or stuck open. That’s the valve located at the charcoal canister. It was covered under the emissions warranty. 

 

FTFY. By the way, P0446 is a circuit code, which points to either wiring or the internal solenoid being shorted/ open. In your case, the solenoid inside was the culprit. 

 

Be aware that some of these trucks have poor wiring management from the factory. The wiring on my '21 wasn't completely encased in the conduit when it branches from the main chassis harness which caused it to chafe on the large conduit causing an open circuit. This was a factory defect due to poor wiring management/ routing.