What does P0456 very small EVAP leak mean?
Short answer: P0456 means the EVAP system failed its test for a very small leak, often a worn gas cap seal, a cracked hose or a slightly leaking purge or vent valve.
The EVAP monitor checks for leaks as small as about 0.020 inch on most late models, which is roughly a pinhole. These leaks do not cause drivability problems, but they keep the check engine light on and fail inspections.
Start with the gas cap and filler neck seal, since they cause many small-leak codes. Then check vent and purge valves; many scan tools can command them to test sealing. Make sure the customer is not topping off the tank past the first click, which can flood the charcoal canister and vent system.
A smoke machine is the right tool for small leaks. Use low-pressure smoke made for EVAP testing, let the system fill, and use a bright light to look for wisps at seams, hose ends and valves. Some machines include a flow gauge that confirms when the leak is found. Never use shop air or an open flame near fuel vapor.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.