Thread: FIXED! - Journey through P0446

I have a 2001 GTP, so the specifics of this post will only apply to the 97-03. I know there are a ton of P0446 threads on this forum but only a few hint at a resolution so I thought I'd write up how I finally fixed mine. I'll start by saying my problem turned out to be my fuel tank pressure sensor. If you know that's not your problem or are curious about how to troubleshoot it, read on. Also, if you're getting this code and it's taking a long time to fill your gas tank or the pump clicks off alot, your issue is most likely your vent valve, vapor canister fouling, or the line that runs between them, so start there.

P0446 is a trouble code that sets when your car's EVAP system fails a pressure test for either high vacuum (indicating a blockage somewhere) or low vacuum (indicating a leak somewhere). The car runs this test basically when it has sat long enough for your engine to come to ambient temperature and there is adequate level in your fuel tank, between 15 and 90% or something like that. The code reads Evaporative Emission Control System Vent Control Circuit Malfunction, but I've come to believe that any of the EVAP codes can be a result of the failure of any component in the system.

The components are:
(1) The gas cap (most commonly blamed component)
(2) Canister EVAP vent valve and solenoid- located behind your fender liner on the in the rear driver side wheel well. Looks like this: Picture
(3) Gas tank and Charcoal Canister- charcoal canister sits right on top of the gas tank, you can just see it with the fender liner removed. Looks like this: Picture (I Know that at least one member has cited this as the actual source of his problem)
(4) Gas tank pressure sensor- located on top of the fuel pump. Looks like this: Picture
(5) EVAP purge valve and solenoid- located on top of the rear valve cover, looks like this: Picture
(6) All of the associated piping and tubing. Filler neck, vent lines, vacuum lines, etc. (I've read a few posts where people found a rusty/pinholed filler neck to the the issue)

A quick google search will tell you the most likely villain involved with P0446 is your gas cap. Be aware, a loose/failing gas cap gasket can throw its own trouble codes, P0455 or P0457 I think. If you're getting both those codes, gas cap is great place to start. Looking for a quick cheap fix, I went ahead and replaced my gas cap anyway. Not the problem, trouble code came right back on after I cleared it on next startup.

After a little more googling/manual reading I decided to replace my evap vent valve and solenoid. It's also pretty cheap and easy to get to; just whip off your back tire, pull out the fender liner and there it sits. 20 minutes later, I had a new vent valve installed. This didn't fix my problem either. In retrospect, this is a pretty easy component to troubleshoot. It is what it describes, a valve that provides a vent path to your vapor (charcoal) canister. With no power applied it's open, with 12V on it shuts. All it takes is a 10mm socket and a pair of pliers to take it off and remove the vent line to the canister. I just wired it up to my car battery, and after I'd put a new one on anyway, could see and hear it shut with power applied. Next.

I stopped by a junk yard and picked up a new Purge valve for $3. It's also very easy to remove and you can troubleshoot it nearly the same way as the vent valve. Pull it off, take the hoses off. Put 12V the valve through the plug blades. If you can blow through a tube into the vacuum-in side with power on, and can't blow through it when you take your 12V off, the valve is good. This valve opens and allows fumes from the vapor canister to pull into the intake, preventing them from entering the atmosphere and burning excess hydrocarbons. At this point I wasn't in the troubleshooting phase though and was just hoping for a quick cheap swap and fix, so I popped the new (gently used) one in. Of course, it didn't work.

Now sh*z getting real. I decide to make my own cheap smoke machine, as described on this forum and youtube in detail. I smoke all my vacuum lines and fuel tank filler neck, and while I find a couple small leaks on my engine, I find nothing that will help me with the code. It's pretty cool though and I do patch up those little leaks. No regrets there. Code still in though.

Multi-meter time...I go back to my vent and purge valves and with the ignition on, probe and back-probe looking for twelve volts on both my little piggies. They're both getting juice, so I know wiring isn't the issue.

Next I look into my charcoal canister. With the fender liner removed, you can just see the end of the charcoal canister with your tubes sitting on top of the gas tank. If you don't know, the vapor canister is full of charcoal pellets that filters/stores the fumes coming from your gas tank and fuel filler neck. It can get fouled or damaged over time from either chronically overfilling your fuel tank, poor fuel quality or physical deterioration. You can finagle off the vent line and vacuum line running to the canister without dropping the fuel tank. If you take off those lines and see a bunch of black crap, like disintegrated charcoal pellets, this is probably your issue. You can also take the vent line off of the vent valve, leave it attached to the canister, and blow through. You should be able to blow with minimal restriction and without getting a ton of black stuff blown back in your face. I was good there, everything looked ok. I also plugged the vacuum line with my thumb and blew into the hose just to make sure the canister wasn't cracked, and it held pressure. So the canister is probably good. If you do have to replace it, a new one is about 90 bucks and you'll have to drop your tank to replace it, but I've read and it doesn't seem like too big a deal.

I'll be honest with you here, the reason I didn't check my fuel tank pressure sensor sooner was that I thought I had to drop the gas tank to check it. As most of you probably already know, at least on the 97-03 models at least there is a little cover in your trunk that gives you direct access. I seriously did a little dance when I found this out. If you drop your middle back seat cover and pull the carpet up in your trunk, there it is. All that stands between you and your fuel pump/pressure sensor is a handful of 10mm "nuts" and a metal plate. The fuel tank pressure sensor plugs directly into the top of the fuel pump and has three wires coming out of it. The gray one on the left should always have 5V on it if your ignition is on, car off. The wire on the right (don't remember the color) shouldn't have much of all on it, but your middle wire is the wire that sends a signal generated by the sensor to the computer. It should never read 0 or 5V, it should be somewhere in between. I back probe this wire with a safety pin and Boom! Zero volts! Problem diagnosed...probably. I've already replaced everything else anyway. This sensor is what actually tells your computer what pressure your fuel tank is at, I.E., whether or not your car passes or fails the little EVAP test it runs. Along with some other stuff I guess.

I ran out to auto zone and paid $50 for a new sensor, plugged in it, checked it, and was getting 1.5V on my sending wire! You can get one cheaper online but I was eager for a win. Cleared the trouble code, has never come back. I honestly believe that I'm getting an MPH or .5 more now too, and a rough idle/surge under acceleration thing I was experiencing has cleared up as well. I'd just accepted them as part of 3.8 life, and them going away may be completely in my head. If you know better, don't say anything, let me have this.

Thanks for hanging in if you've read all this, I hope that it helps someone save some of the money/time I spent on this stupid trouble code that shouldn't exist anyway.