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OBD fault code guide

P0455 Code Meaning UK

P0455 means “Evaporative Emission Control System Leak Detected — Large Leak”. In plain English, the fuel vapour system has a leak big enough that the engine computer cannot seal the EVAP system during its self-test. Common causes include a loose or missing fuel cap, disconnected vapour hose, split EVAP pipe, cracked charcoal canister, leaking filler neck, faulty purge valve or faulty vent valve.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 EVAP large leak fault
Quick answer Fault codes hub Small EVAP leak
✓ P0455 meaning ✓ Large leak checks ✓ UK repair costs ✓ MOT-aware advice
Quick answer Meaning Symptoms Causes Diagnosis Large leak checks Repair costs Can you drive? MOT impact Related guides FAQs
Quick answer

What does P0455 mean?

P0455 means the ECU has detected a large EVAP leak. The EVAP system should be able to seal the fuel tank vapour system during a self-test. If it cannot hold pressure or vacuum at all, or the leak is larger than expected, the ECU stores P0455. A missing or loose fuel cap is one of the simplest causes, but a disconnected EVAP pipe or cracked canister can cause the same code.

Most likely area

Fuel cap, filler neck, vapour hoses, charcoal canister, purge valve, vent valve or tank vapour lines.

Urgency

Moderate if there is no smell, urgent if there is strong petrol smell or visible fuel leakage.

MOT risk

Can affect warning light status, emissions checks and fuel system safety.

Code meaning

P0455 — EVAP System Large Leak Detected

The full description is usually P0455 Evaporative Emission Control System Leak Detected Large Leak. This is different from P0442, which normally points towards a small leak. P0455 suggests a bigger opening in the EVAP system or a component that cannot seal at all.

Large leaks are often easier to find than tiny leaks, but they still need proper testing. A missing fuel cap, detached hose or broken canister connection can trigger the code quickly.

Mechanic view

Why P0455 can be simple or serious

Sometimes P0455 is as simple as a fuel cap left loose after filling up. Other times it is a split vapour hose, broken filler neck, cracked canister or vent valve stuck open.

The key is smell and safety. If there is a strong petrol smell, do not treat it like a harmless warning light. Fuel vapour and fuel leaks need proper checks.

Symptoms

Common symptoms of a P0455 code

P0455 often does not affect how the engine drives, but it can create obvious fuel vapour symptoms.

Engine management light

A steady engine warning light is the most common symptom.

Strong petrol smell

A large vapour leak can cause a noticeable petrol smell near the filler, tank or engine bay.

Fuel cap warning

Some vehicles may show a fuel cap or emissions system warning.

No driveability issue

The car may drive normally because the fault is in the vapour system.

Hard start after refuelling

A purge valve fault can sometimes cause difficult starting after filling the tank.

EVAP readiness failure

The EVAP monitor may not complete until the large leak is repaired.

Common causes

What causes a P0455 fault code?

P0455 is commonly caused by a large vapour leak, open valve, missing cap or disconnected EVAP pipe.

Very common

Loose or missing fuel cap

If the fuel cap is not fitted correctly, the EVAP system cannot seal.

Very common

Damaged fuel cap seal

A badly cracked or missing seal can create a large vapour leak.

Hose fault

Disconnected EVAP hose

A hose that has popped off or been left disconnected can trigger P0455 quickly.

Vapour line

Split vapour pipe

A large split in a vapour line can stop the system holding pressure or vacuum.

Filler neck

Leaking filler neck

Rust, cracks or damaged filler neck seals can cause a large EVAP leak.

Canister

Cracked charcoal canister

A damaged canister or broken pipe connection can create a major vapour leak.

Valve fault

Purge valve stuck open

A purge valve that cannot seal can cause EVAP test failure.

Vent system

Vent valve stuck open

The system may not seal if the vent valve cannot close during testing.

Tank area

Fuel tank vapour leak

Leaks around tank seals, pump seals or vapour ports can trigger large leak codes.

Diagnosis

How a garage should diagnose P0455

✅ Scan all stored, pending and permanent codes.

✅ Check whether P0440, P0442 or P0446 are also present.

✅ Inspect the fuel cap first and confirm it seals properly.

✅ Check the filler neck for rust, cracks or cap seating damage.

✅ Inspect EVAP hoses for disconnected or split pipes.

✅ Command purge and vent valves closed where possible.

✅ Smoke test the EVAP system to locate the large leak.

✅ Inspect charcoal canister, tank seals and vapour line connections.

✅ Check EVAP pressure sensor data during leak testing.

Avoid this mistake

Do not keep driving with a strong petrol smell

A large EVAP leak can sometimes be only vapour, but a strong petrol smell should always be taken seriously. Check for visible leaks, wet fuel marks and dripping before assuming it is harmless.

If the smell is strong inside the cabin or around the car, stop using the vehicle until it has been checked.

Large leak checklist

Mechanic-style checks for P0455

A P0455 diagnosis should quickly separate simple fuel cap faults from larger leaks around the tank, canister or EVAP valves.

Cap fitted correctly?

Confirm the cap clicks/tightens properly and the seal is not missing.

Filler neck condition

Check for rust, cracks, damaged threads or poor cap seating.

Hose disconnected?

Look for vapour hoses that have popped off near the canister or tank.

Smoke escaping fast?

A large leak should usually show quickly during smoke testing.

Valve sealing

Purge and vent valves must close properly during EVAP leak tests.

Canister damage

Check for cracked plastic, broken nipples or damaged pipe connections.

UK repair cost guide

How much does P0455 cost to fix in the UK?

Costs vary depending on whether the fault is a loose cap, broken hose, valve issue, canister damage or tank-area leak. These are rough UK guide prices only.

Diagnostic scan

Usually around £40–£120 depending on how much EVAP testing is included.

Fuel cap replacement

Often one of the cheapest repairs if the cap seal is faulty or missing.

Smoke test

Useful for confirming where a large vapour leak is escaping.

EVAP hose repair

Can be modest if the damaged hose is easy to access.

Purge or vent valve repair

Usually moderate cost depending on valve location and part price.

Canister or filler neck repair

Can cost more if the fault is near the tank or requires extra access.

Safe driving advice

Can you drive with a P0455 code?

If the car runs normally, the engine light is steady and there is no strong petrol smell, short careful driving is usually possible. But because P0455 means a large vapour leak, you should check the fuel cap and look for fuel smell or visible leaks before continuing.

✅ Lower risk: steady engine light, no petrol smell, no visible leak and normal driving.

⚠️ Medium risk: fuel smell near the filler, repeated EVAP codes or hard start after refuelling.

🚫 Higher risk: strong petrol smell, visible fuel leak, dripping fuel or petrol smell inside the cabin.

Real experience

What I would check first

On a real P0455 job, I would start with the fuel cap. Is it fitted? Does it click? Is the rubber seal damaged? It sounds basic, but it is one of the most common large-leak causes.

If the cap is fine, I would smoke test the system and inspect the canister and tank-area pipes. A large leak usually shows itself quicker than a small P0442 leak.

MOT implications

Will P0455 fail an MOT?

P0455 can lead to an MOT failure if the vehicle has an emissions-related engine warning light, strong fuel smell, visible fuel leak, poor emissions or obvious fuel system safety concerns. Any visible fuel leak should be treated as urgent.

Engine warning light

An emissions-related engine light should be investigated before MOT.

Strong fuel smell

A strong petrol smell can indicate a significant vapour or fuel system fault.

Visible fuel leak

Visible fuel leakage is a serious safety issue and should be repaired immediately.

Engine light MOT guide Common MOT failures
Back to main hub

Browse more OBD fault code guides

P0455 is one common EVAP large leak diagnostic trouble code. For more common codes, meanings, symptoms, repair costs and MOT implications, use the main Motor Vehicle Expert fault-code hub.

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Frequently asked questions

Frequently Asked Questions About P0455

Find answers to common questions about P0455 EVAP large leaks, petrol vapour faults, driving safety, repair costs and MOT implications.

What does P0455 mean?

P0455 means the engine computer has detected a large leak in the EVAP system. In simple terms, the fuel vapour system cannot seal properly and the leak is bigger than a small seep.

Can I drive with a P0455 code?

You can often drive carefully with P0455 if the car runs normally and there is no strong petrol smell. If there is a strong fuel smell, visible fuel leak or fuel dripping, stop driving and get it checked.

Will P0455 fail an MOT?

P0455 can lead to an MOT failure if it causes an emissions-related engine warning light, strong fuel smell, visible fuel leak, poor emissions or an obvious fuel system safety concern.

What causes a P0455 code?

Common causes include a missing, loose or damaged fuel cap, disconnected EVAP hose, split vapour line, cracked charcoal canister, leaking filler neck, faulty purge valve, faulty vent valve or fuel tank vapour leak.

How much does P0455 cost to fix in the UK?

A diagnostic check may cost around £40 to £120. A fuel cap or simple hose repair may be modest, while smoke testing, canister, filler neck, purge valve or vent valve repairs can cost more depending on access.

Is P0455 worse than P0442?

Usually yes. P0442 normally means a small EVAP leak, while P0455 means a large EVAP leak. P0455 is more likely to involve a loose cap, disconnected pipe or larger vapour leak.

About this guide

Based on practical mechanic-style diagnostic experience

Motor Vehicle Expert publishes practical UK vehicle guidance based on real-world mechanical knowledge and hands-on diagnostic experience. This P0455 guide is written to help drivers understand EVAP large leak faults before replacing parts unnecessarily.

Fault codes should always be treated as a starting point for diagnosis. If your car has a strong petrol smell, visible fuel leak, fuel dripping, rough running or a flashing engine light, stop driving and get professional help.