P0455 or P0442 Diagnosis

How to find an evaporation system leak.
How does eva p system work: when you add fuel thru the filler pipe or the tank heats up during the day, the vent valve is open so fuel vapor goes out the tank vent line and gets absorbed in the charcoal canister. When you drive, the purge valve opens and draws fuel vapor from the canister to be burned in the engine.
Every once in a while on the highway, the computer closes the vent valve
while monitoring the fuel tank pressure sensor to run a leak test. A GM computer will open the purge valve to pull a vacuum on the tank and then close it and watch for pressure increase. A Chrysler computer will keep the purge valve closed and turn on the air pump to pressurize the tank and then watch for pressure to decrease. If the pressure changes a lot, the computer sets P0455. If the pressure changes a little, the computer sets P0442.
If there are additional related codes, these can help a lot with diagnosis. If you have a purge valve or vent valve code (P0443, P0446, etc.)-maybe that valve isn’t closing as necessary during the leak test. Suspect these and check next by removing and checking if they close. Purge valve defaults closed and should not allow flow when unplugged. The vent valve defaults open and must be closed with 12vdc and blown into to check for flow.
If there is a pressure sensor code (P0452, 453, 454), the test will not run and the computer will likely set a leak code, as it cannot verify the seals are not leaking.
A well--equipped shop will disconnect the vent line at the canister and blow smoke into the vent line to see where it comes out. If you don't have a smoke tester, blow into the vent line with your lungs. If air keeps going in, you have a leak (have an assistant listen for the leak at filler cap and above the fuel tank, etc.) Alternately, blow into the canister side of the vent path while blocking the vent valve outlet (note that in some vehicles the vent valve is near the air fliter as it vents into the engine with inlet air). If the canister leaks, check for cracks in the canister or flow into the purge line. If no leaks observed in canister, you can disconnect the purge line and blow in it to check the purge line and valve for leaks. If you have a Chrysler/Jeep or other vehicle with an air pump connected to the evap system, disconnect the pump line and blow towards the pump to check for leaks in the pump and pressurization line. Also perform a visual inspection of the system for obvious leaks. If the leak is above the fuel tank, it will be hard to find without smoke. On Chrysler/Jeep/BMW, it's possible that the pump may not be producing the produce spec pressure. You can run the pump manually by applying voltage with the vent valve closed while watching the fuel tank pressure sensor live data using an OBD scan tool.
Yes, these are a lot of things to check. If the fuel cap is good, the diagnosis is difficult.
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Components that may fail:
Fuel cap seal or not tightened
Purge or vent valve stuck open (remove and check operation using 12vdc and lungs)
Canister cracked
Purge, vent, return lines or seals at ends leaking.
Fuel filler pipe leak
Ford - no cap seal leaking
Notes: large leak can usually be found. Small leak (different code) harder. Doesn’t affect vehicle operation but codes prevent registration renewal in some states.