With all leak detection pump operated systems, the engine vacuum (or pump vacuum in this case) is sent back to the leak detection pump, it uses this vacuum and lifts and then releases a diaphragm in the LDP and that is used to "pump" air pressure into the fuel tank. (Yes, they use vacuum(negative pressure) to push and pull a tiny pump and it creates positive pressure...sounds weird but thats how it works!). When enough pressure is built in a calculated amount of time, the system seals and watches the pressure for decay. If no decay, its a PASS and in 2 consecutive passes, (on 2 consecutrive cold starts, not 1) the EVAP MONITOR will complete. If the system leaks a small amount but still holds for a short time, youd get a SMALL leak code. If the system cannot MAKE any pressure, or it makes the pressure but it leaks out too quick to measure, it sets a LARGE leak code.
On most LDP vehicles, the pumps rarely fail (older chryslers, 1996-2004 notwithstanding, the old ones actually do fail kinda often)...LDPs dont fail very often. much more common is cracked hoses/lines, loose orings for lines, or a physical leak in the tank area (fuel cap or fuel module seal)...and not uncommon are leaking purge valves up by the intake.
The best way to test is with smoke. You can seal off the tank by activating the vent control valve with a scanner or a jumper wire to close the tank vent (exits under the rear fender liner near the fillneck, large rubber hose leaving the LDP and heading to a plastic cylinder that has a filter in it but otherwise is just an exit port). Pinch the hose completely shut with some pliers, of disconnect it and plug it with something tight (a large bolthead or a cork or similar that fits). Now, the only exit for your fuel vapors is the fuel filler (which should have a tight gascap with a good uncracked rubber seal) and the purge line. disconnect the purge line up front (On gen 2 cayenne its the larger plastic pipe that comes up mid firewall, then has a connector with a release button near the right head, then from the connector it runs down the channel between the valve cover and intake towards the purge valve in the front). When the purge is OFF (not currently commanded to purge!) there should be NO VACUUM IN THAT HOSE, You can tell if the purge is ON/PURGING because the valve in the front will be CLICKING. If there is any vacuum when its NOT clicking, its probably bad. shut off the engine, it should ALWAYS turn OFF. You cannot blow through a closed/off purge valve on any car. So engine off, disconnect the line and see if you can blow through it.
If the purge passes make sure you retest it a few tymes, as they can be intermittant and only fail when warm or only leak 1 out of 10 tries and set the light on...then you test it 5x in a row and it always seems to pass but it can still be bad...so test it several tymes before declaring it good(and they actually do fail alot is why I say that.), If it is absolutely good...then comes the smoke testing. ONLY use an approved evap smoke machine. They pump baby oil into a very low temp heater (about 90f degrees) and the baby oil starts turning to smoke. at 90f, there is no danger of igniting gasoline. If you wanna try using a cigar or a party smoker...understand, they are made to use FLAME in a cigar tip...or heaters that get HOT, or temperature dependent smoke sources (dry ice say) which will stop smoking as soon as they traven more than a few feet or which will stay hot and when they hit the gas fumes in the tank...GOOD LUCK!
Now that we have the safety points...youd put smoke into the lines heading back to the tank. WIth the vent hose pinched, and smoke put into the PURGE line (The larger line at the firewall with the plastic disconnect button on its fitting)...no smoke should be able to get out. NONE. Not even a small wisp...it is only good if its NONE. If any smoke comes out, find it (flashlight helps) and fix it! bad line, bad Oring, bad tank seal...whatever...find and fix.
If no smoke comes out...and if you have a flowmeter on the evap smoker (most do) and it says no flow after a minute or so...you have no PHYSICAL LEAKS. Now you look for MECHANICAL LEAKS. IE - something that is supposed to seal when told to to create the pressure...and isnt sealing to allow the pressure for the leak test to reach required levels. If the LDPs vent solenoid is the leak (it can be)....the smoke will exit the vent line even though its closed with the scantool or activating with a jumper wire...but would stop when you pinch the hose closed with needlenose pliers. If you dont have a scantool and cant figure out the jumperwire idea...then check VACUUM. The SMALLER hose heading back from the intake...its a vacuum hose, it doesnt have a button release like the larger hose...and it comes out the center of the intake, then makes a big horseshoe turn rught by the top center of the firewall before clipping into the bracket and heading down and back side by side with the larger hose that feeds the purge line. Yank it off of its plastic connector (its simply pushed on) and put smoke in IT back to the tank, if your flow check shows no flow after a few seconds...its not leaking. If it shows flow, or you see smoke...find and fix. straight forward stuff as discussed above. If it is NOT leaking, now you wanna measure vacuum at the vacuum line which goes to the vacuum pump. In most cases, simply idle the engine and pulle the vacuum hose off. The pump on the right head always makes vacuum when the engine is running. That hose is connected to it, so you should have good solid constant vacuum on it. if there is no vacuum...then the pump isnt making vacuum or the line is off or pinched. If the pump has failed...youd have no vacuum to anything on the car that uses vacuum...IE power brakes! So if the car also has brake problems...you likely have your answer. If the brakes are perfect and you ONLY have a p0455, then id check to make sure the vacuum to the LDP in that smaller line is stable and present and over 10 inches. Remember, a LARGE LEAK code is set when pressure leaks out so fast it cannot be measured as holding at all. However, if no pressure is ever even created (like that small hose being broken or disconnected so the LDP never makes any pressure or the vacuum pump being bad) the PCM doesnt know the difference and will call it a LARGE LEAK when in fact the problem was the vacuum to the LDO was never applied to begin with...and the fuel tank system in fact has NO LEAKS.
Hope it helps. And yes, this logic applies to all LDP evap vehicles with some small mods in scanner control and locations of lines. But the above works on a 2008 cayenne specifically.