So I've had this code since about 2011 and finally decided to get around to fixing it recently. I pulled the fender liner out since I had seen a video of someone showing the solenoid on the bottom of the charcoal canister being what gets replaced and that fixes the evap problems. I ordered what I thought was that solenoid, and then saw that it wasn't at all the same. It ended up being the one on the side of the airbox (077-133-517-c). But, while I was in there, I noticed that the two wires that run down to the bottom-of-canister solenoid were actually broken where it goes through the rain tray.
I cleaned the wires up and stripped them back and put some butt-splices in there to get it reconnected. Plugged everything in, put it all back together, cleared all the codes, and I thought I had fixed it once and for all, since the CEL didn't come back on and all eight readiness checks passed.....for about 60 miles, then CEL came back on for evap slow leak again.
edit: I also have replaced the gas cap.. twice.. with a OEM one..not some aftermarket knock-off.
Then I actually replaced that solenoid on the side of the airbox, cleared the codes, and all was fine...again, for about 60 miles, then it came back. Checked all the lines today looking for dry rot or cracks/tears and everything is fine. All the clamps are tight. I decided to remove my butt-splice job and replace that with soldered wires instead, since I had it all torn apart.
I had seen in a video before, someone had their car up on a rack with the engine running, and they showed that bottom-of-canister solenoid hanging freely and it wasn't doing anything. Unplugged it and plugged a new one in, and it started clicking like crazy, and they said "that's how you know if it is bad or not." Mine wasn't clicking at all.
Took the solenoid out of the canister, hooked some alligator clips up to the battery and touched the contacts on the solenoid.. it clicks and moves as it should. So the solenoid itself isn't bad. Before I soldered the patch into the wiring, I stripped the original wires back enough so that I could touch those directly to the contacts on the solenoid, and even then, it still didn't work.
I grabbed the Bentley and went to 97-391 and couldn't decide if it was N112 or N80, but either way....both go back to the ECU. So... the solenoid isn't being cycled by the ECU for some reason.
What I'm wondering is... A) does the engine have to be up to full operating temp before the evap system starts doing anything? B) if there's already a CEL and a code for the evap system, does the evap system get disabled most of the time until the ECU decides to run a self-test on it again once in a while?
While I was in there, I noticed that on the side of my canister, "Nov 20 04" is ink-stenciled on there, but the solenoid in it has 96T290 (guessing day 290 of year 96--which matches everything else on the car that is still factory). So I'm guessing the canister got replaced at one point, but the old valve was carried-over. I had no codes when I bought it in 2006 and codes were very rare and seldom until about 2011.
I looked in ETKA, and the canister does not show or even have a solenoid on the bottom of it, but Bentley manual shows one there and even has it labeled. When I punch in "1H0-906-518-A" (what is stamped onto the solenoid) to ETKA, part not found. ECS Tuning and 1stvwparts both come back with saying that is a "flap" and neither have a picture for it, and both say that it is discontinued, there's no more stock anywhere for it, and there is no replacement.
I'm... I'm at the limit of my knowledge on this specific system at the moment. I've heard before that you can't get the solenoid by itself--that it only comes with a new canister, but it is weird that ETKA doesn't even show that there could be a solenoid on the bottom of the canister. I know the diagrams in there are not definitive, but it is weird that the solenoid has a part number, but the part number doesn't seem to exist.