P0456: Evaporative Emission System Leak Detected (very small leak)
The computer found a very small EVAP leak, around 0.020 inch, which is about the size of a pinhole.
What P0456 means
P0456 is the smallest leak the EVAP self test is designed to catch, roughly 0.020 inch on most newer vehicles. The engine computer seals the system and watches the tank pressure sensor for a slow drift. Because the leak is so small, outside factors such as hot weather and fuel that is boiling off can affect the test, so the computer only runs it under specific conditions, often after the car has sat overnight. Common causes are a worn gas cap seal, a hairline crack in a hose or canister, and a purge or vent valve that does not seal completely. These are hard to spot by eye, so shops use a smoke machine that pushes visible vapor into the system at low pressure, then look for wisps with a bright light. Some leaks only open up when the tank is warm or under the right pressure. P0456 does not affect how the car drives, but you will likely need it fixed to pass an emissions inspection.
Common symptoms
- Check engine light on
- Usually no change in how the car drives
- Faint fuel smell near the car or the fuel door
- Failed emissions or readiness test
- Other EVAP codes may appear at the same time
Most common causes
- Worn gas cap seal
- Tiny crack in an EVAP hose or fitting
- Purge or vent valve that does not seal fully
- Hairline crack in the charcoal canister
- Leaking fuel pump or sending unit seal
- Faulty fuel tank pressure sensor
What to check yourself first
Replace an old or worn gas cap with a quality part that matches the original. Make sure it clicks. Then clear the code and give the car a week of normal driving, since the test only runs under specific conditions.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| EVAP smoke test to find the leak | $61-$90 | A smoke test is the standard way to find EVAP leaks. |
| Replace gas cap (shop price) | $138-$150 | A new cap is a cheap first step for pinhole leaks. |
| Replace EVAP purge valve | $210-$251 | If the purge valve leaks slightly when closed. |
| Replace EVAP canister (includes vent valve on many cars) | $558-$643 | For a cracked canister or leaking vent valve. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0442 Evaporative Emission System Leak Detected (small leak)
- P0455 Evaporative Emission System Leak Detected (large leak)
- P0457 Evaporative Emission System Leak Detected (fuel cap loose/off)
- P0440 Evaporative Emission System
- P0451 Evaporative Emission System Pressure Sensor/Switch Range/Performance