I've been chasing down this one quite awhile. The ellusive "Evap emission control system leak detected - gross leak". Like Ahab pursuing his whale. I think I finally cornered him. A 'net search will reveal many hits, with all sorts of suggestions, some easy, some not. All depends where the leak is. I've tried all of the simple ones; no joy. I'd clear the code and it would come back in a few days. I'd check/fix something else, clear the code; "I'M BACK!!!!" it would show it's orange face on my dash. I'd checked my gas cap, purge valve, lots of vacuum hose, charcoal canister, check valves, everything thing I could reach. No joy. "I'm BAAAAACK" I didn't have a complete understanding of this system. It stretches from the radiator to the back hatch. My mechanic has a scanner capable of running a diagnostic test on this system, activating the valves in the correct sequence and displaying the tank pressures in real time. Problem is the numbers go by quite fast. It confirmed the PO455 code. It also confirmed that my tank pressure sensor was working. It also confirmed that the purge & shut-off valves were opening and closing. But that was it. I was greeted with a large FAIL at the end. This is obviously a vacuum leak, probably affecting my mileage, which is bothersome given the price of gas. So this time around, I decided to plan my attack. I watched several youtube videos on troubleshooting these sytems which was very helpful. Don't usually even think about looking up a problem there. Also found a few videos that actually explain how this system works in general. Many mentioned using smoke to track down the leaks. Maybe I could figure out a way to 'blow smoke'! I also found a LARGE picture on line of the tiny vac. diagram stuck to the underside of my hood. (Post picture here) Another tool I used this time around is a tiny airbrush compressor that can also supply vacuum. Trying to pull much of a vacuum on a car system with a hand vac pump is frustrating to say the least. You need a bigger pump. But not too big. I began by disconnecting the purge valve, which is attached to the radiator fan shroud on my car, and hooking the line leading back to the char. can up to the pressure side of the pump. Then I went around listening, making my way to the back of the car, while laying on my back. Nothing. I finally reached the charcoal cannister at the other end of the line, and found a hard-plastic 90 deg. fitting with a weird connector on it. I figured out the connector, removed it and air was hissing out. I plugged it up with my finger and heard no other hissing. So no leaks on this line between the back of the car & the front. OK, one down. I removed the char. cann & all but one hose connecting it to the shut-off valve. I then pulled a vacuum directly on the inlet line, while plugging the other ports. I could see the sides of the box pull in, so I let go. WHOOOOSH. I switched the fittings on the pump so now I had pressure. Same result: No leaks. OK, no leaks in the can or attached hose (the infamous J-hose). Three down. I finagled a way to attach the pump the the tank vent hose, which had another hard-plastic weird connector on it. I just so happen to have the right OD sized piece of soft-silicone vac hose that when shoved up in the connector, made a nice, tight fit. Attached the free end to the vac side, made sure the gas cap was snug, and turned on the pump. After running 15 sec, I loosened the gas gap to a loud WHOOOOOOSH! Haven't heard that sound in a LONG time. Swapped to the press. port and repeated. "WHEEEEEEESSSSHHHHH" OK. So I can rule out the tank, the tank vent hose, fuel pump & sender gaskets, that's seven down. I pulled a vac again. Turned off the pump (the pump valves will maintain press or vac . . up to a point) Let it sit for 20 minutes. Unscrewed the gas cap . . . nothing. Hmmmm. OK. Vac'd again, this time waited 6 min. "WHOOOOOSH" Ok so the tank won't hold a vac for 20 min., but it will for 6. Good enough for now. So I'd checked everything but the valves. I left these for last as I'd checked the purge valve (on the radiator shroud) numerous times and it held a vac. I'd checked the shut off valve (near the char can) once before and it was fine. Thus I thought my devil lay elsewhere. But where? I started with the shut-off valve. This valve allows air to flow through it both ways when it's not energized. IOW, it's N.O. So it must be energized first to check it. Hooked it up to the battery, pulled 12"Hg on the port in-line with the electrical connector and watched the gauge for 2 min. No drop. Did the same on the perpedicular connector. Wouldn't go any higher than 5"Hg, then quickly dropped to zero. Retested. Same result. So this one is suspect. The in-line port connects to the char can. The perp. port connects to an air filter. Since this port is NO, air flows through it, into the char can. mixes with tank vapors, and then is drawn out by engine vac when the purge valve (NC) is opened. When energized, the shut-off valve is capable of holding a strong vac on its in-line port, but not so capable on its perp port. But then again, it doesn't need to. This valve is closed to seal off the vent system when a test is performed, so it could be the cause of a GROSS LEAK if it leaks. BTW 15"Hg = .5 bar = .5 atm = 7.4psi = 204" water One more thing about the shut-off valve. It's either ON or OFF only. Not pulsed to my knowledge. I spends the majority of its life OFF. Next up was the purge valve. This valve is N.C. with no power. I taped both ports on the concrete. Nothing fell out. I'd read that sometimes this valve gets clogged by free charcoal bits from the cann. (I even tapped my char. can on the ground, ports down: Nothing came out. No free carbon! I pulled 10"Hg on the in-line port. Held fine. Did the same on perpendicular port, wouldn't hold pressure. Repeat. Same result. Gave it a few taps and sprayed some silicone into both ports, taped out the excess, then repeated. This time the perp. port held fine. Retested the in-line port. Fine. Next I repeated this test with power applied, as this valve is pulsed by the ECM. I used 1/2sec on/off cycles, listening to hear if the valve synch'd in time to voltage. After about 10 of these, it would miss every so often. Not sure what the duty cycle of the signal from the ECM could be, but this one wasn't very regular with even 1/2sec.ON 1/2 sec OFF. After this cycling, I rechecked each port with the vac pump and no power; In-line was the same as before, holding fine (this port goes to the intake manifold); However the perp. port would only go to 10", then immediately drop to 5" then slowly to 0". I repeated this several times. The purge valve should hold vac from both sides. This one fails that. It's Toes Up. Some have reported that these valves are faulty even if they test OK. (????) Well I have two here that won't hold any vacuum on their perpendicular ports. They're also 12 yrs old. So I'm going to order new ones. First I'll replace the purge valve, reset, then drive and see if PO455 arrises. If so, in will go the shut-off valve. I've learned something about troubleshooting vacuum leaks: They're worse on turbocharged cars! I could have thrown easy parts at this, many do. But enough occurances of pinched fuel pump gaskets, rotted hoses on top the tank (which must be dropped for access), cracked hard-lines from bumping parking stops, defective check valves, rotted J-hose, and on and on told me it's better to diagnose, then prescribe else the cure may elude you. Yea, this post if long. But it's a condensed version of many hours in the driveway in 102F heat! I hope it helps someone else diagnose their CEL-throwing ellusive vac leak. I'll report back when I get the new parts installed.