P0456: A small EVAP leak can be detected after the engine is switched off

Ford's P0456 strategy evaluates a nominal 0.020-inch EVAP leak using engine-off natural pressure and vacuum behavior. The test can continue after key-off, which explains why running the engine at idle is not necessarily a useful attempt to verify this particular monitor.

The preceding drive enables the parked test

The source requires eligible thermal history, fuel level, ambient conditions and sufficient operation before shutdown. During key-off, refueling-like pressure or level changes, unstable tank pressure, low battery voltage or turning the ignition back on can abort evaluation.

A test that stops for one of those reasons has not proved the system tight. Ford lists a maximum 45-minute evaluation when a fault is present, although successful checks may complete sooner. That is a monitor description, not a recommendation to repeatedly cycle the ignition or deliberately force a leak test.

Read both pressure and vacuum evidence

The published malfunction criterion combines insufficient pressure build and insufficient vacuum build. Mode $06 therefore exposes separate positive-pressure and negative-pressure results, plus a normalized aggregate on applicable systems.

Ford warns that zero values may be defaults for phases that have never completed. Check completion status before interpreting a zero as a pass. The code identifies inadequate sealed-system behavior under the test conditions; it does not locate the leak or prove that a fuel cap, purge valve or canister is the failed component. Vehicle-specific leak diagnosis remains necessary.

Source and applicability

This explanation covers the cited Ford monitor variants, not every vehicle displaying this code. Match the engine, sensor architecture and calibration before applying numerical limits.

Ford OBD summary, PDF page 14, Ford OBD summary, PDF page 15, Ford OBD summary, PDF page 16.