Cause
Blocked or pinched vent hose
The evaporative system's vent hose crushed or kinked, so air cannot enter or leave the tank. It is the mechanical version of a blocked vent path, and its position under the vehicle is what causes it — a jack placed on the line, a kerb strike, or a heat shield refitted across it.
A blocked vent puts the tank into vacuum as fuel is used. The pump works against it, delivery suffers as the tank empties, and there is a distinct rush of air when the filler cap is opened — which is the confirming test and needs no equipment at all.
It also breaks the leak monitor, since the system cannot draw the air it needs to run its test, so a blocked vent and a leak both produce evaporative codes from the same area. Disconnect the hose at both ends and blow through it — a definitive test in seconds. Check the routing against its clips afterwards, since a hose that has been displaced will be pinched again by whatever displaced it.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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