OBD-II code · P0455
On this page
- What the code actually means
- Symptoms
- Is it safe to drive?
- What causes it — most common first
- How to diagnose it, in order
- 1. Check the fuel cap — 30 seconds
- 2. Inspect the fuel cap gasket
- 3. Visual inspection of vapor lines
- 4. Smoke test if visual inspection finds nothing
- 5. Test the vent valve / CCV
- Fixes, cheapest first
- How to reset the code after a repair
- What to do if it comes back
- Related OBD-II codes
P0455 Code: Evaporative Emission Control System Leak Detected (Gross Leak)
What the code actually means
SAE J2012 defines P0455 as "Evaporative Emission Control System
Leak Detected (Gross Leak)". The PCM seals the evap system during a
test cycle and watches pressure bleed off. If the leak rate matches a
0.040-inch (1 mm) or larger orifice, this code sets.
P0455 is the largest standard evap leak code. Compared to its
siblings:
P0440: Generic evap fault — no size info.P0442: Small leak (about 0.020 inch).P0455: Gross leak (about 0.040 inch and larger).P0456: Very small leak (about 0.010 inch).
A leak this large is usually obvious — the fuel cap is missing, a hose
fell off, or the vent valve is stuck wide open. P0455 is a
two-trip code on most platforms.
Symptoms
- Check engine light on, usually the only symptom.
- Faint gasoline smell at the fuel filler.
- Failed state emissions test until the code clears.
Is it safe to drive?
Yes, indefinitely. P0455 does not cause engine damage or
drivability problems. The evap system stores and burns fuel vapor; a
gross leak means vapor escapes to atmosphere instead. Daily driving is
fine; an emissions inspection will not pass.
What causes it — most common first
1. Missing or loose fuel cap (~60%). A cap left off after a fill-up, or a cap that doesn't click 3 times. Clue: code appeared shortly after fueling; cap visibly loose or has a damaged gasket.
2. Cracked, disconnected, or split evap hose (~25%). A vapor hose detached from the canister or split at a clamp. Common after a fuel pump replacement or muffler service. Clue: visible disconnection on inspection.
3. Stuck-open vent valve / CCV (~10%). The canister close valve froze open or its plunger failed. Clue: PCM cannot pressurize during self-test; smoke from the canister vent line.
4. Cracked charcoal canister (~3%). Wheel-well impact or rodent damage. Clue: visible crack in the canister shell.
5. Filler-neck failure (~2%). Rust pinholes around the welds where the filler tube connects to the tank. Clue: common on rust-belt vehicles over 15 years old; fuel smell stronger near the tank.
How to diagnose it, in order
1. Check the fuel cap — 30 seconds
Open the fuel door and confirm the cap is present and tight (3+ clicks
to torque). About 60% of P0455 codes resolve in 30 seconds at this
step.
2. Inspect the fuel cap gasket
Pull the cap. Examine the rubber O-ring on the underside. Cracks, hardening, or a flattened groove mean the seal is done. Replace with an OEM cap ($15–$30). Universal aftermarket caps frequently fail to seal on ORVR-compliant systems.
3. Visual inspection of vapor lines
With the engine off, follow visible vapor hoses from the filler neck to the canister, and from the canister to the purge valve at the intake. Look for:
- Hoses pulled completely off (most common gross-leak cause).
- Large cracks or splits.
- Crushed lines from prior repairs.
4. Smoke test if visual inspection finds nothing
A smoke machine introduces low-pressure smoke. With a gross leak, smoke billows out the leak point within seconds — much faster and more obvious than the small-leak version. Cost: $60–$120 at most shops.
5. Test the vent valve / CCV
Command the vent valve closed with a scan tool. Apply 5 in-Hg of vacuum at the canister; pressure should hold for 30+ seconds. If it bleeds off immediately, the vent valve is stuck open.
Fixes, cheapest first
| Fix | Cost (USD) | When it applies |
|---|---|---|
| Tighten or install the cap | $0 | Cap was loose or missing |
| Replace the fuel cap | $15–$30 | Gasket cracked; cap won't click |
| Reconnect or replace a hose | $5–$50 | Visual inspection found it |
| Replace vent valve / CCV | $30–$150 | Valve stuck open on test |
| Replace charcoal canister | $80–$300 | Physical crack visible |
| Repair filler-neck rust | $200–$800 | Rust-belt vehicle, pinhole confirmed |
The cap-first approach is what every shop tries before quoting smoke service. Don't skip it.
How to reset the code after a repair
Clear with a scan tool, then complete 2–3 full drive cycles under evap monitor conditions: fuel between 1/4 and 3/4, ambient 40–95 °F, mixed driving, 8+ hours engine-off between cycles.
Without a scan tool, expect 200–400 miles before the readiness flag sets.
What to do if it comes back
- Within a day of replacing the cap: the cap was a wear item, not the root cause. Inspect or smoke-test the rest of the system.
- After every fix: filler neck or fuel tank pinhole, usually on a rust-belt vehicle older than 15 years.