P0455

A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.

Safe to drive

Safe to drive. Tighten the fuel cap first. If you smell strong raw fuel or see liquid fuel, stop and have it inspected.

Urgency

Not urgent

Will it pass emissions

No

If ignored

The light stays on, which masks any new code appearing behind it, and the vehicle fails OBD emissions inspection. Fuel vapor vents to atmosphere instead of being burned. A broken vent path also lets dirt and water reach the charcoal canister, turning a cheap hose repair into a canister replacement.

Check engine light

Solid

Clearing the light

A completed drive cycle is normally required before the monitor re-runs and the light can clear.

• Possible smell of fuel near tank or engine bay
• Failed emissions inspection
• Loose-cap warning may appear

Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.

These are the causes this code can have. For the order to work through them in, see what to check first.

Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.

In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.

1. The fuel cap — present, correct for the vehicle, seated, and clicking

   Free, immediate, and the actual fix a large share of the time. A cap left on the roof at a gas station is a textbook P0455.

2. The cap's rubber seal and the filler neck sealing surface

   A hardened, cracked or missing O-ring, or rust and debris on the neck, leaves a gap big enough for the monitor to score as gross.

3. EVAP hoses around the canister and the fuel tank

   Canisters usually sit under the vehicle near the tank, where road debris and rodents reach them. Look for a hose knocked off, crushed, or chewed through.

4. Whether this vehicle uses a capless filler system

   Many newer vehicles have no removable cap at all. The seal is then a flapper assembly inside the filler neck, and the failure mode is a stuck or damaged flapper, not a loose cap.

5. Any recent work near the fuel tank, canister, exhaust, or rear suspension

   A quick-connect left off after tank, pump, or suspension work is one of the most common causes of a P0455 that appears suddenly.

1. Inspect, seat and test the fuel cap and filler neck

   Tools Known-good cap, light

   Confirm a cap is present and is the correct part for the vehicle. Remove it, inspect the O-ring for hardening, cracking or missing material, wipe debris and rust from the filler neck sealing surface, and refit until it clicks. Fit a known-good cap if one is available. On a capless system, inspect the flapper assembly instead.

   Expected

   Cap sealing cleanly on a smooth, undamaged neck, or an intact flapper on capless systems.

2. Read fuel tank pressure sensor live data and confirm it responds

   Tools Scan tool with EVAP live data

   Watch the sensor with the cap off, then with the system sealed and vacuum applied. A sensor that reads flat regardless of conditions is itself a candidate and would prevent the monitor from ever passing.

   Expected

   Reads near atmospheric with the cap off, and moves when the system is sealed and vacuum applied.

3. Trace and inspect the entire vapor path

   Tools Light, jack and axle stands

   Follow the lines from the filler neck to the tank, to the charcoal canister, through the vent valve, and forward to the purge valve at the engine. Look for cracked plastic lines, brittle rubber connectors, unseated quick-connects, and rodent damage. Support the vehicle properly on stands before going underneath.

   Expected

   Continuous, intact lines with every connection fully seated.

4. Smoke test the EVAP system

   Tools EVAP smoke machine, bidirectional scan tool for valve control

   This is the definitive test. With the vent valve commanded closed, introduce low-pressure smoke at the service port or through the vent line, then look for escaping smoke along the entire path. Use only an EVAP-rated smoke machine at low pressure — the system is not built for shop-air pressure.

   Expected

   No visible smoke escape. Smoke at the filler neck, a canister crack, or a hose joint locates the leak directly.

5. Command the vent and purge valves and confirm they seal and open

   Tools Bidirectional scan tool, hand vacuum pump

   With bidirectional control, close the vent, open the purge, and watch tank pressure fall. A vent valve stuck open never lets the system pull down at all — the classic P0455 pattern with no crack anywhere in the system.

   Expected

   Vacuum builds when the vent closes and purge opens, then holds once both are closed.

6. Inspect the charcoal canister itself

   Tools Visual inspection

   Look for cracks, impact damage from road debris, and saturation with liquid fuel. A canister flooded by chronic overfilling at the pump can crack, restrict, and also cause hard starting immediately after refueling.

   Expected

   Canister body intact, not impact-damaged, and not soaked with liquid fuel.

7. Check the filler neck and, on capless systems, the flapper seal

   Tools EVAP smoke machine, light

   Rust or a bent neck prevents even a new cap from sealing. On capless systems, smoke the neck with the filler door closed and watch for escape around the flapper.

   Expected

   No smoke escaping at the neck with a known-good cap or an intact flapper in place.

8. Repair, then let the monitor run before declaring success

   Tools Scan tool with readiness monitor status

   After the fix, clear the code and drive normally with the tank between roughly a quarter and three quarters full, including at least one overnight cool-down for systems that use natural vacuum.

   Expected

   EVAP readiness monitor reports complete and passing, and the code does not return.

This is one of the few codes where the fix is genuinely often under $50. Have the cap checked before paying for anything else. If the cap is not it, expect $100 to $180 for a proper smoke test, which usually finds the leak in a single visit. Canister and filler neck jobs cost more mostly in labor, because the tank area is awkward to reach and often requires the vehicle on a lift.

• Replacing the fuel cap, clearing the code, and declaring victory before the monitor has run again. It can take days and specific conditions.
• Pressurizing the system with shop air. It damages the canister and the valves.
• Habitually filling the tank past the pump's first click. Overfilling pushes liquid fuel into the charcoal canister and eventually ruins it.
• Assuming every EVAP code is the gas cap. P0455 is a large leak — a merely worn cap seal usually sets P0442 or P0456 instead.
• Overlooking the vent valve. A valve that will not close reads as a wide-open system even though nothing is cracked or disconnected.
• Fitting a cheap aftermarket cap that does not meet the original sealing specification.

• Fuel Filler Cap
• EVAP Canister
• EVAP Hose

• P0454EVAP System Pressure Sensor/Switch A Circuit IntermittentSeverity level 3 of 5: Diagnose soon.moderate
• P0456Evaporative Emission System Leak Detected (Very Small Leak)Severity level 2 of 5: Not urgent.low

Definition

Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.

Diagnostic guidance

Written and reviewed in-house. See our editorial policy.

Review status

Reviewed

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