Code explained

P0456: a very small leak in your fuel vapor system

P0456

In plain words: The fuel vapor system has a very small leak.
Standard name: Evaporative Emission System Leak Detected (Very Small Leak)
System: Fuel vapor (EVAP) system

General information, not a mechanic's diagnosis: have a mechanic confirm before any repair.

What the computer saw

The fuel vapor (EVAP) system keeps gasoline fumes from the tank out of the air and sends them to the engine to burn. To test it, the computer seals the system and watches the pressure, usually after the car has sat for some hours. If the pressure creeps back slowly, there is a leak, and when the leak is the size of a pinhole the computer sets P0456. It is the smallest leak the test reports: P0442 is a small leak and P0455 a large one.

Likely causes, most common first

A gas cap that does not seal. A worn seal, a cap that was not tightened until it clicks, or a cap that does not fit the filler.
A purge or vent valve that does not close fully. These valves open and close the system for the test; one that leaks a little lets the pressure creep back.
A cracked hose or a worn filler neck seal. Rubber hoses and seals harden and crack with age, often in places hard to see.
A cracked charcoal canister or fuel tank fitting. Less common, and usually found with a smoke test.

What you can note yourself

Whether the gas cap was tightened until it clicks after the last fill-up, and whether its rubber seal looks cracked.
Whether the light came on soon after a fill-up.
Every stored code: a P0442 or P0455 with it points at the same system.

What to tell the shop

All the codes, including any pending ones.
Ask whether they ran a smoke test to find the leak before replacing parts.
Whether you already replaced the gas cap, and when.

This page gives no verdict on whether a car is safe to drive; a mechanic can.

Questions people ask next

Can a gas cap cause P0456?

Yes. A cap that does not seal is a common cause, which is why it is worth checking first. A new cap does not always clear the code, because the valves and hoses can leak too.

Why does P0456 take days to come back after it is cleared?

The computer runs the leak test only under certain conditions, such as after the car has sat for some hours with the fuel level in a set range. It can take several days of normal driving before the test runs again.

What is the difference between P0456, P0442 and P0455?

They are the same test with different leak sizes: P0456 a very small leak, P0442 a small one and P0455 a large one. Smaller leaks are usually harder to find.