P0455 on a 2018 Subaru Outback
P0455 means the EVAP system failed a leak check badly enough that the computer sees a large leak or no pressure/vacuum change.
Not your vehicle? See general P0455 info.
Most common causes on the 2018 Subaru Outback
Ranked from 805 NHTSA owner complaints filed for this vehicle.
- 1
EVAP purge or vent valve stuck, producing a large detectable leak
Medium severityThe same owner-reported venting failure (pressurized tank, slow fill) is large enough to trip the large-leak (P0455) threshold as well as the small-leak code.
Reported in 1 NHTSA complaint - 2
Rodent-damaged EVAP or fuel line causing a large leak
High severityChewed-through fuel/vapor lines create a leak path large enough for the large-leak monitor to fail.
Reported in 1 NHTSA complaintParts:EVAP hose/line - 3
Disconnected or split EVAP hose
Medium severityA hose that has popped off a fitting or split open creates an obvious, large vapor leak.
Common cause - 4
Loose or missing gas cap
Low severityA cap left loose or unsealed is large enough to fail the large-leak EVAP monitor on the next drive cycle.
Common causeParts:gas cap - 5
Cracked charcoal canister
Medium severityA significant crack in the canister housing lets vapor escape well beyond the small-leak threshold.
Common causeParts:charcoal canister - 6
Corroded or damaged fuel filler neck
Medium severityA rusted-through filler neck creates a sizeable, continuous vapor leak.
Common causeParts:fuel filler neck
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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0455 on your car still needs shop confirmation before any repair.
Common symptoms
- Check engine light
- Fuel smell
- Usually no performance issue
- Failed emissions readiness
What to check before replacing parts
Check the gas cap first
Look for: A loose, missing, or cross-threaded cap is by far the most common cause of a large EVAP leak code — if it doesn't click to full tightness or the seal is cracked, that's very likely the whole fault.
Inspect visible EVAP hoses
Look for: A large leak is often obvious: a hose disconnected at a fitting, split open, or chewed through (rodent damage is common) rather than a hairline crack.
Command the purge and vent valves with a scan tool
Look for: A valve that doesn't respond at all when commanded open/closed, or stays open when it should seal, prevents the system from holding pressure/vacuum and reads as a large leak.
Smoke-test the system
Look for: With a large leak, expect an obvious plume of smoke or a clearly audible hiss at the fault location — it should be easy to spot compared to a small-leak diagnosis.
Inspect the canister and filler neck
Look for: A cracked charcoal canister housing or a corroded/rusted-through filler neck (common on older vehicles) can leak enough to prevent the system from ever holding vacuum.
Rather have a pro handle it? Find a vetted local mechanic.