Possible causes

– Missing or loose fuel cap
– Incorrect fuel filler cap used
– Fuel filler cap remains open or fails to close
– Foreign matter caught in fuel filler cap
– Evaporative Emission (EVAP) canister or fuel tank leaks
– Evaporative Emission (EVAP) system hose leaking
– Fuel tank leaking

When is the code detected?

Evaporative Emission (EVAP) control system has a very large leak such as fuel filler cap fell off, EVAP control system does not operate properly.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Possible a noticeable fuel odor caused by the release of fuel vapors

P0456 SUBARU Description

This diagnosis detects leaks in the Evaporative Emission System (EVAP) purge line using engine intake manifold vacuum. If pressure does not increase, the Engine Control Module (ECM) will check for leaks in the line between the fuel tank and EVAP canister purge volume control solenoid valve under the following vacuum test condition.
The vacuum cut valve bypass valve is opened to clear the line between the fuel tank and the EVAP canister purge volume control solenoid valve. The EVAP canister vent control valve will then be closed to shut the EVAP purge line off. The EVAP canister purge volume control solenoid valve is opened to depressurize the EVAP purge line using intake manifold vacuum. After this depressurization is implemented, the EVAP canister purge volume control solenoid valve will be closed.