Sharing my experience with P0440 code on my GS400. Dreaded CEL code P0440 only. Consistently got code every second trip despite resetting. Hard fault of some kind. Replaced gas cap twice including new Stealer cap. Thoroughly checked everything under the hood, purge hoses and purge solenoid. Under car checked all canister hoses, lines, etc. No problems. Removed 3-way solenoid and bench test OK. Tapped into the ECB connector pins and was able to check the vapor pressure sensor input and output while sucking on hose. Stop laughing. So assumed it was a problem with the tank vent valve. Actually took my car to the lexus dealer, first time ever, and paid $130 for them to confirm the tank vent valve was "bad". They quoted $780 to replace the vent valve, the grommet seal, and the plastic vent line, parts and labor. So I bought the parts on-line for about $80 and then the fun started. Actually, replacing the vent valve was not that bad, 2 hours start to finish. Drop the exhaust pipes and mufflers in one piece, take off a couple heat shields, take off the driveshaft, unbolt the parking brake brackets and a couple other pieces, and take off the bolts on the front of the straps holding the tank. I had driven the car until the fuel light was on, then drove about 75 more miles to make sure the tank was basically empty. So the tank was not heavy. I did not disconnect the fuel pump, filler neck hose, or any wires. I dropped the tank down about 5 inches until I could reach up from the front and get at the vent valve. It's just plugged into a grommet. So I pried out the vent valve using a screwdriver, but the POS valve broke into two pieces and of course one piece fell into the tank. Ouch! So the broken piece is in the tank, no big deal. I installed the new vent valve, grommet, and vent hose, and bolted everything back. Problem fixed, no more CEL from hell after several trips. BTW, the plactic vent line that goes from the vent valve to the canister is a total POS. Shame on Toyota engineers. Almost impossible to remove without damaging, in fact the lexus tech said they always replace that POS hose. And grease up the grommet to make installation of the vent valve easier.