Cause
Blocked EVAP vent path
The path by which the evaporative system draws fresh air, blocked so the tank cannot breathe. The vent is how the system equalises — fuel drawn out must be replaced by air, and purging pulls air through the canister — so blocking it disables both.
The distinctive symptom is a tank that goes into vacuum. A blocked vent means the pump draws against a sealed tank, so fuel delivery suffers as the tank empties and there is an audible rush of air when the filler cap is opened. That hiss on opening the cap is the confirming evidence and costs nothing to check.
The vent is deliberately open to atmosphere, which is what fouls it: mud, road spray, insects and spiders nesting in an open pipe. Its filter is the other common blockage. Check the vent valve and its filter with the line disconnected, and be aware a vent valve stuck closed produces the identical symptom with a clear path — commanding the valve while watching flow separates the two.
Typically a moderate job.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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