Emission codes
P0456: evaporative emission system very small leak
P0456 is the smallest leak the fuel vapor system checks for, a hole of about half a millimeter. It is a frustrating code because it can come and go with the weather.
P0456: can I drive, and what will the labor cost?
Estimated labor, replace the gas cap
$11 to $22
| Labor time (estimate) | 0.1 to 0.2 h |
| Rate used | $110/h |
| Technician wage, United States (average) | $27.13/h |
| Low: fix when convenient Safe to drive; the leak is tiny, and finding it usually takes a smoke test. | |
Labor only: parts and the diagnostic fee are not included. Hours are our estimate for a typical car; book times vary by model. Wage: BLS, May 2025.
P0456 is the generic OBD-II code for "evaporative emission system leak detected, very small leak". Federal rules adopted the California requirement that the system find a leak as small as 0.020 inch, about half a millimeter (40 CFR 86.1806-17). At that size, the test is sensitive to everything that changes vapor pressure: fuel temperature, fuel volatility, the fuel level and the weather. That is why P0456 can appear, disappear and come back over weeks. The most frequent fix is still the gas cap seal, which wears and hardens. A purge valve that does not seal completely, a pinhole or loose clamp on a vapor hose, and a vent valve whose seal leaks a little are the others. It is safe to drive and has no effect on how the car runs. It does keep the light on, and in OBD-tested states, a stored code fails the inspection, so it is worth a methodical search rather than parts at random.
After the repair: when does the light go off?
Light off by itself after about
2 days
| Fault-free trips needed | 3 |
| Warm-up cycles before the code can be erased | 40 |
| Stored code gone after about | 40 days |
Federal rule for model years up to 2016 (40 CFR 86.1806-05); a misfire or fuel fault must also stay away under similar conditions. Clearing codes with a scanner also resets the readiness monitors an inspection checks.
Codes, rules and labor data checked by Radif Partners · Editorial policy · Method and sources
P0456 at a glance
| Code | P0456 |
| Standard description | Evaporative Emission System Leak Detected (Very Small Leak) |
| System | Evaporative emissions (EVAP) |
| Severity | Low: fix when convenient |
| Common symptoms | none |
Why the weather matters
The test watches pressure change in a sealed system over a set time. On a hot day, fuel evaporates and pressure rises; on a cold day, it falls. The computer allows for this, but at the very small leak threshold the margin is thin, and a marginal seal can pass one week and fail the next. A code that only shows up in one season suggests a part that seals when warm and leaks when cold, often a rubber seal.
A methodical search
- Replace the gas cap with the correct one for the car. If the code does not return within about two weeks of varied driving, you are done.
- If it returns, ask for a smoke test with the vent closed and the system at the low test pressure. Very small leaks need patience and, sometimes, a UV dye.
- Have the purge valve tested: a valve that lets vapor through when it should be closed can look like a leak.
Newer cars, stricter tests
From model year 2017 onward, the federal rules incorporate California's 0.020-inch requirement and name P0456 among the generic codes for it. Older cars may have tested only to 0.040 inch (P0442). Compare with P0455, the opposite end of the scale, where the leak is large.
Likely causes of P0456, in order
| Cause | How often |
|---|---|
| Gas cap seal | most frequent |
| Purge valve not sealing fully | frequent |
| Pinhole or loose clamp on a vapor hose | frequent |
| Vent valve seal | less frequent |
P0456 repairs and labor time
| Repair | Labor (estimate) | At $110/h |
|---|---|---|
| Replace the gas cap | 0.1 to 0.2 h | $11 to $22 |
| EVAP smoke test to find the leak | 0.5 to 1 h | $55 to $110 |
| Replace EVAP purge valve | 0.3 to 1 h | $33 to $110 |