P0456: Evaporative Emission System Leak Detected (Very Small Leak)
Title: SAE J2012 generic definition. Manufacturer-specific definitions and enhanced codes can differ; check service information for the exact vehicle.
What P0456 means
The EVAP system failed its leak test with a very small leak, the hardest EVAP leak to find.
What sets it
The very-small-leak test is more sensitive than the P0442 small-leak test. California's OBD II rules describe the target as a leak equal to a 0.020-inch hole (general guideline only; the manufacturer's calibrated threshold applies). Because the leak is tiny, temperature and fuel level affect the test, and the monitor typically runs only under specific conditions.
Symptoms
- None apart from the light
- Inspection failure
Is it safe to drive?
General guidance, not a guarantee: Safe to drive in normal circumstances. It is an emissions fault, not a drivability fault. If in doubt, have it towed and diagnosed.
Common causes
Listed roughly from most to least common in general practice. The real order varies by vehicle, mileage and history.
- Gas cap seal
- Hairline hose cracks or tired O-rings
- Purge or vent valve seat seepage
- Filler neck seal or small rust pinholes
- Canister
Test before you replace: diagnostic sequence
General guidance only. Always follow the vehicle manufacturer's service information, specifications and safety procedures. Any figures below are general guidelines that defer to the manufacturer's specification.
- Inspect and test the cap; many cap testers can check cap sealing.
- Smoke-test with patience: seal the system, leave the smoke in for several minutes, and inspect with a strong light. Very small leaks may only show as a faint wisp.
- Pay attention to valve seals and O-ring connections, not just hoses.
- Use the scan tool's EVAP leak test (where available) to confirm the system holds after repair.
- Explain to the customer that the monitor may need specific conditions and several days to confirm.
Bench note: Set expectations with the customer: a very small leak can take real time to find and confirm. Quote the diagnostic time up front.
How-to guides for this code
Related codes
FAQ
P0456 vs P0442: what's the difference?
Leak size. P0456 is the very small leak test (0.020-inch target in California rules); P0442 is the small leak test (0.040-inch target).
Why can't the shop find the leak?
Very small leaks are genuinely hard to see. A methodical smoke test with plenty of time is the normal approach.
Will it affect fuel economy?
Not noticeably. It mainly affects emissions and inspection.
Sources
- SAE J2012, Diagnostic Trouble Code Definitions (standard page; code titles only)
- California Code of Regulations, 13 CCR 1968.2, OBD II requirements (via Cornell LII)
- California Air Resources Board, On-Board Diagnostic (OBD) program
Written with AI assistance and reviewed for accuracy; not a substitute for OEM service information. Shop Docs Studio is not affiliated with SAE, EPA, CARB or any vehicle manufacturer.