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OBD fault code guide

P0442 Code Meaning UK

P0442 means “Evaporative Emission Control System Leak Detected — Small Leak”. In plain English, the engine computer has found a small fuel vapour leak in the EVAP system. It is often caused by a loose fuel cap, damaged cap seal, tiny split in a vapour hose, leaking filler neck, purge valve not sealing, vent valve problem or charcoal canister leak.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 EVAP small leak fault
Quick answer Fault codes hub EVAP system fault
✓ P0442 meaning ✓ Small leak checks ✓ UK repair costs ✓ MOT-aware advice
Quick answer Meaning Symptoms Causes Diagnosis Smoke test Repair costs Can you drive? MOT impact Related guides FAQs
Quick answer

What does P0442 mean?

P0442 means the ECU has detected a small leak in the EVAP system. The EVAP system is designed to trap fuel vapour from the fuel tank and burn it through the engine later. A small leak can be as simple as a fuel cap not sealing, but it can also come from a cracked vapour hose, filler neck seal, purge valve, vent valve or charcoal canister.

Most likely area

Fuel cap seal, vapour hose, filler neck, purge valve, vent valve, charcoal canister or tank pressure sensor.

Urgency

Usually moderate if there is no fuel smell, but should be repaired to clear the emissions fault.

MOT risk

Can affect engine warning light status, emissions checks and fuel vapour safety concerns.

Code meaning

P0442 — EVAP System Small Leak Detected

The full description is usually P0442 Evaporative Emission Control System Leak Detected Small Leak. The important part is “small leak”. The ECU has run an EVAP self-test and found that the system cannot hold pressure or vacuum as expected.

Small EVAP leaks can be difficult to see by eye. A pipe may look normal but leak under test. That is why proper smoke testing is often more useful than visually guessing.

Mechanic view

Why P0442 can be annoying to diagnose

A P0442 leak can be tiny. The car may drive perfectly, use fuel normally and show no obvious sign apart from the engine light. That does not mean the code is not real — it means the leak is in the vapour control side of the fuel system.

On a real job, I would check the fuel cap first, then smoke test the system rather than throwing purge valves or sensors at it.

Symptoms

Common symptoms of a P0442 code

P0442 normally causes fewer driving symptoms than engine or turbo faults, but it can still keep the engine warning light on.

Engine management light

A steady engine warning light is the most common symptom.

Petrol vapour smell

You may notice a faint fuel smell near the filler neck, rear of the car or engine bay.

Fuel cap warning

Some vehicles may display a fuel cap or emissions warning message.

No change in driving

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

Readiness monitor issue

The EVAP monitor may fail to complete until the leak is repaired.

Hard start after refuelling

If the purge valve also sticks, the engine may be harder to start after filling up.

Common causes

What causes a P0442 fault code?

P0442 is usually caused by a small vapour leak or a valve that does not seal properly during the EVAP self-test.

Very common

Loose fuel cap

A cap that is not fully tightened can allow vapour pressure to escape.

Very common

Damaged fuel cap seal

A cracked, flat or missing rubber seal can cause a small EVAP leak.

Vapour leak

Tiny split in EVAP hose

Small cracks in vapour pipes can leak enough to trigger P0442.

Filler neck

Filler neck leak

Rust, damaged seals or poor cap seating around the filler neck can cause small leaks.

Valve fault

Purge valve not sealing

A purge valve that leaks when closed can cause EVAP test failure.

Vent system

Vent valve not sealing

The vent valve must close properly during system tests.

Canister

Charcoal canister leak

A cracked canister or damaged pipe connection can allow vapour loss.

Sensor

EVAP pressure sensor issue

Incorrect pressure readings can make the ECU think there is a leak.

Previous repair

Disconnected vapour pipe

A pipe disturbed during fuel tank, engine or repair work can cause a small leak.

Diagnosis

How a garage should diagnose P0442

✅ Scan all stored, pending and permanent codes.

✅ Check whether P0440 is also present.

✅ Inspect the fuel cap and rubber seal carefully.

✅ Inspect the filler neck for rust, damage or poor cap seating.

✅ Check EVAP hoses for small splits or loose connections.

✅ Command purge and vent valves where the diagnostic tool allows.

✅ Smoke test the EVAP system to locate small leaks.

✅ Check EVAP pressure sensor data during testing.

✅ Avoid replacing parts until the leak location is confirmed.

Avoid this mistake

Do not clear the code and ignore it

P0442 often returns after a few drive cycles because the EVAP test does not run constantly. Clearing the light may look like it worked, but the ECU will retest the system and turn the light back on if the leak is still there.

Fix the leak, then let the vehicle complete its readiness checks properly.

Small leak diagnosis

Why a smoke test is useful for P0442

Because P0442 is a small leak, the leak may be too small to hear or see. A proper EVAP smoke test puts smoke into the vapour system and shows where it escapes.

Fuel cap area

Smoke may escape around the cap seal or filler neck.

Hidden hoses

Small splits can hide above the tank, near the canister or under the car.

Valve sealing

A purge or vent valve may leak internally even if it looks fine outside.

UK repair cost guide

How much does P0442 cost to fix in the UK?

Costs vary by leak location, access and whether the fix is a simple cap, hose, valve or canister issue. These are rough UK guide prices only.

Diagnostic scan

Usually around £40–£120 depending on the garage and testing depth.

Fuel cap replacement

Often one of the cheapest repairs if the cap seal is damaged.

EVAP smoke test

May cost extra, but it is one of the best tests for small leaks.

Small hose repair

Can be modest if the leaking hose is accessible.

Purge or vent valve repair

Usually moderate cost depending on part location and access.

Canister or tank-area repair

Can cost more if parts are above the tank or underneath the car.

Safe driving advice

Can you drive with a P0442 code?

If the car runs normally and there is no strong petrol smell, short careful driving is usually possible. P0442 is not normally as urgent as a misfire, oil pressure warning or overheating fault. But fuel vapour faults should still be repaired because they affect emissions and can sometimes point to a fuel system leak.

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

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

🚫 Higher risk: strong petrol smell, visible fuel leak, dripping fuel or fire-risk concern.

Real experience

What I would check first

On a real P0442 job, I would start with the fuel cap and filler neck. Many small leak faults come from simple sealing problems, especially if the cap is old or the seal is hard.

If the cap is good, I would smoke test the EVAP system. Guessing small leaks without smoke can waste a lot of time because the leak may be hidden above the fuel tank or near the charcoal canister.

MOT implications

Will P0442 fail an MOT?

P0442 can lead to an MOT failure if the vehicle has an emissions-related engine warning light, a strong petrol smell, visible fuel leak, poor emissions or fuel system safety concerns. A small EVAP vapour leak may not affect how the car drives, but it can still affect the emissions system.

Engine warning light

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

Petrol smell

Fuel smell can point to a vapour leak or fuel system issue.

Visible fuel leak

Any visible fuel leak is a serious safety issue and must be repaired.

Engine light MOT guide Common MOT failures
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Browse more OBD fault code guides

P0442 is one common EVAP small 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 P0442

Find answers to common questions about P0442 EVAP small leaks, fuel vapour faults, driving safety, repair costs and MOT implications.

What does P0442 mean?

P0442 means the engine computer has detected a small leak in the EVAP system. The EVAP system captures fuel vapour from the fuel tank and stores it so it can be burned by the engine instead of escaping into the air.

Can I drive with a P0442 code?

You can usually drive carefully with P0442 if the car runs normally and there is no strong petrol smell. However, the fault should still be repaired because it can affect emissions and may indicate a fuel vapour leak.

Will P0442 fail an MOT?

P0442 can lead to an MOT failure if it causes an emissions-related engine warning light, strong fuel smell, visible fuel leak, poor emissions or obvious fuel system safety concerns.

What causes a P0442 code?

Common causes include a loose or damaged fuel cap, small split in an EVAP hose, cracked vapour line, filler neck leak, purge valve not sealing, vent valve problem, charcoal canister leak or EVAP pressure sensor issue.

How much does P0442 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, purge valve, vent valve or canister repairs can cost more depending on access.

Is P0442 serious?

P0442 is usually not as urgent as a misfire or overheating fault, but it should not be ignored. A small EVAP leak can keep the engine light on and may point to a fuel 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 P0442 guide is written to help drivers understand small EVAP leaks 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, rough running or a flashing engine light, stop driving and get professional help.