Cause
Incorrect non-original fuel cap
A replacement fuel cap that does not meet the vehicle's specification. The cap is a pressure-rated component, not a lid — the evaporative system runs the tank at a slight pressure or vacuum, and the cap must both seal against that and vent at defined thresholds.
A generic cap seals well enough to look right and fails the test. Its seal may be the wrong compliance, its vent valve the wrong pressure, or it may vent when it should hold — so the monitor reports a small leak on a cap that is undeniably tight. A leak code with a brand new cap fitted is this, and it defeats the obvious repair.
Some caps are also part of the fuel filler's own interlock, so a non-original one can leave a sensor unable to confirm the cap is present. Check the part number against the vehicle rather than the fitment guide, and be aware that a cap from a related model in the same range is frequently the wrong rating despite fitting perfectly.
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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