I'm getting a P0455, large leak in the evap system. However, it only looks like it triggers when the evap system readiness check happens. I can reset the code and drive for days with multiple startups with no problems until readiness sets and I get the code. Using a durametric, it looks like the system is operating correctly as the tank pressure difference reading has pressure slowly building up to -20 during idle and then going back towards zero on throttle. I haven't changed the gas cap yet but it looks good with no cracks. Date code on it is 2018 and previous owner also chasing the same issue said he changed to a new OEM one. The tank pressure difference leads me to believe there aren't cracks in the lines, evap canister, or the gas cap.

I've tested the fuel line vent valve. I think it's working correctly but am not sure. Blowing through the black opening provides some resistance then relief after a certain amount of pressure. Blowing through the orange opening has very little resistance before air goes through. Pressing on the plunger, fully opens the valve and it is clear both ways. I read somewhere that the red side should have more resistance. Can anyone confirm?

I've also tested the evap purge valve. It holds positive pressure but can't really hold a vacuum. Is this functioning correctly? I read that some evap systems will conduct the readiness test with a vacuum pull and hold at idle but again am not sure if this is the case and don't have another evap purge valve to test on.

Hoping to get more information rather than buying and trying parts. Sort of annoying since the car runs fine, filling up gas is fine with the nozzle upside down, but the MIL goes on whenever evap sets readiness which is typically a few days at the very least.

 

Went through some checks and for reference, it looks like it's the evap canister vent solenoid, also known as the shutoff solenoid. Activated in the durametric and didn't hear any clicking. Verified voltage at the plug. Gave the solenoid a couple of whacks and it kicked back in. Have a new one on the way which will hopefully cure the code.