Number P0456: EVAP System Leak Detected (very small leak)
The evaporative emission (EVAP) system stores fuel vapour from the tank in a charcoal canister; a purge valve lets the engine draw it off, and a vent valve opens the canister to fresh air. P0456 reports a very small leak of 0.020 in (0.5 mm), as CARB requires. Ford uses the engine-off natural vacuum
Can you drive with P0456?
Severity: Plan the repair. Yes — this is an emissions-system fault. The car will generally drive normally, but it will fail an emissions inspection with the light on.
If you ignore it: Fuel economy and emissions creep up, and some faults in this family (rich/lean running) stress the catalyst over time.
This is the #28 most common code in our top-150 library, in the evap — fuel vapor system family (Powertrain system). Severity is Number Nest's editorial triage — if the light is flashing or the car feels unsafe, stop driving it whatever any page says.
What P0456 means
The evaporative emission (EVAP) system stores fuel vapour from the tank in a charcoal canister; a purge valve lets the engine draw it off, and a vent valve opens the canister to fresh air. P0456 reports a very small leak of 0.020 in (0.5 mm), as CARB requires. Ford uses the engine-off natural vacuum test: after switch-off the closed system must build at least about 190 Pa pressure or 125 Pa vacuum, judged statistically. Saab's Bosch ME9 also tests with the engine off for up to 45 minutes; VW and Audi use a leak detection pump. GM reports this leak size as P0442 instead. The code often appears only after many trips and depends on the weather.
Systems involved: evap system, fuel filler cap, evap hoses, evap leak detection pump.
Causes, ranked by likelihood
| # | Cause | Likelihood |
|---|---|---|
| 1 | Fuel cap seal worn or cap not clicked in | most likely |
| 2 | Purge or vent valve not sealing | possible |
| 3 | Porous hoses or O-rings, hairline crack in canister or filler neck | possible |
| 4 | Leak detection pump itself leaking (VW/Audi) | less common |
Check them in order — the list is ranked so the likely, cheaper causes come before the expensive part.
Symptoms you may notice
- Check engine light on (possibly a check fuel cap message)
- Possibly fuel smell
- EVAP readiness or emission inspection not passed
Misdiagnosis traps
- Very-small-leak codes are the hardest EVAP faults: parts-swapping almost never wins. A smoke test with the system sealed — and time — is the repair.
- Aftermarket gas caps cause a surprising share of repeat very-small-leak codes; the seal geometry differs from factory.
How a good shop confirms it
- Factory cap first if yours is aftermarket, then smoke-test with soapy water on the filler neck and canister seams.
Repair options & costs
| Repair | Shop total (parts + labor) | DIY (parts only) | Difficulty |
|---|---|---|---|
| EVAP leak diagnosis (smoke test) + gas cap | $73 – $248 | $10 – $60 (parts only) | 1/5 |
National ranges from Number Nest's cost model (midsize car, independent shop, US-average labor).
OBDex repair data for this code rates the job difficulty medium with a typical labor time in the 0.2–3 hour range and DIY possible: yes.
Price this repair in your state
Estimated total: — (midsize car, independent shop)
Frequently asked questions
Why does my mechanic want to keep the car overnight?
The EVAP monitor often runs after a cold soak, and very small leaks show best cold. Overnight is diagnosis, not delay.
How much does it cost to fix P0456?
The fix most associated with this code — evap leak diagnosis (smoke test) + gas cap — runs $73 – $248 at an independent shop for a midsize car in our cost model (parts + labor, US average). The code names a system, not a part, so confirm the cause before spending: the ranked causes above are ordered by likelihood.
Can I drive with P0456?
See the can-you-drive verdict above — it is based on the system involved and the cascading-damage risk, and it is our editorial triage, not a manufacturer instruction. If the light is flashing or the car feels unsafe, stop driving it regardless of any code.
Will P0456 clear itself?
If the fault was a one-off (a loose cap, a cold-start glitch), the light can go out after several clean trips and the code ages into history. A fault that is still present re-sets the code as soon as the car runs that self-test again — monitors run under specific conditions, which is why "it hasn't come back in two days" is not proof of a fix.
Will my car fail inspection with P0456?
In states with OBD-based emissions inspection, yes — an illuminated check-engine light or an incomplete readiness monitor fails the test regardless of what the tailpipe would show. C-codes (ABS) fall under safety inspection instead in states that have one.
What should I check first for P0456?
The first item on the ranked-causes list above — it is ranked likelihood-first for exactly this reason — plus the free checks: recent work in the area (connectors left loose are a classic), fluid levels, and the battery and grounds on electrical codes.
Where this data comes from
Code facts, causes and symptoms come from OBDex — an open dataset of independently authored descriptions of the standardized generic OBD-II codes (CC0 1.0, public domain; github.com/foerbsnavi/OBDex). The SAE J2012 document itself is copyrighted and is not reproduced here. Cost ranges are Number Nest's own US planning model: national parts and labor ranges cross-checked against the CarMD Vehicle Health Index 2026 published averages, with labor adjustable by the BLS OEWS May 2025 state wage index for auto technicians (occupation 49-3023) — a wage index, not a surveyed shop rate. Severity and can-you-drive notes are our editorial triage, not a manufacturer instruction.