P0442 Fault Code: What It Means and What To Do

What does P0442 mean?

P0442 means the engine management system has detected a small leak in the evaporative emission (EVAP) system. The EVAP system is a sealed network of hoses, valves, and a charcoal canister that captures petrol vapours from the fuel tank and stores them until the engine can burn them. The ECU tests the system by sealing it off and checking whether it holds pressure — if it detects even a small leak, it logs P0442.

"Small" is relative — this code is specifically for leaks of around 0.020 inches or less. A larger leak would trigger P0455. P0442 is one of the most common engine warning light triggers, and in a significant proportion of cases the cause is simply a loose, cracked, or worn fuel cap.

Common causes

• Loose or faulty fuel cap — the most common cause. If the cap isn't sealing properly, the system cannot hold pressure. Always check this first.
• Cracked or split EVAP hose — the small rubber and plastic hoses connecting the fuel tank, charcoal canister, and purge valve can crack with age, particularly in cold climates.
• Faulty purge valve (purge solenoid) — this valve controls when vapours are drawn from the canister into the engine. A leaking or stuck-open purge valve causes a continuous leak in the system.
• Faulty vent valve — the vent valve allows fresh air into the system during the purge cycle. A leaking vent valve also breaks the seal.
• Cracked charcoal canister — less common, but the canister itself can crack, especially if it has been exposed to water or road debris.

Typical UK repair costs

UK labour rates are typically £80–£120 per hour. Costs vary by vehicle and region.

What to do next

Step one: tighten the fuel cap until it clicks. Drive for a few days and see if the light goes out — the ECU re-tests the EVAP system automatically. If the light clears, the cap was the problem. If it comes back, try a new fuel cap (cheap and worth ruling out).

If a new cap doesn't fix it, a mechanic with a smoke machine can inject smoke into the EVAP system and quickly identify the exact location of the leak. This is much faster than guessing at hoses or valves blindly.