P0455 Code: Large EVAP Leak Detected

What does P0455 mean?

P0455 stands for "Evaporative Emission System Leak Detected (Gross Leak)." Gasoline gives off vapor in the tank. The EVAP system stores that vapor in a charcoal canister and later feeds it to the engine to burn. It is a sealed system, so the engine computer can test it for leaks.

On many vehicles, the test works like this: the computer closes the vent valve, uses engine vacuum through the purge valve, then watches the tank pressure sensor. Some designs use a small pump instead, but the idea is the same. If it cannot pull or hold vacuum, the system is open somewhere. ALLDATA, a repair information company, puts it simply: the code means the EVAP system has detected a large leak somewhere in the system ^[1].

Detecting leaks is a federal requirement. The U.S. EPA lists the evaporative system among the systems OBD II must monitor, and notes that a missing fuel cap should be detected by the evaporative system check ^[2]. P0455 is the EVAP code for the biggest leaks, which is why a missing or loose cap is the first suspect.

Is it safe to drive with P0455?

You can usually drive the car, but treat P0455 as a "fix soon" code, not one to live with. Carista, a maker of diagnostic apps, lists the symptoms as the check engine light, a faint fuel smell near the rear of the vehicle, and usually no drivability problems ^[5].

Here is why we put it above the smaller EVAP leak codes:

• Fuel vapor is escaping. A large opening lets gasoline vapor out of the system every time the car sits, and that is the pollution the EVAP system exists to stop.
• A strong fuel smell changes the answer. If you smell strong raw fuel or see fuel dripping, do not keep driving: park away from heat and sparks and have it checked first. That points to a leak of liquid fuel, not just vapor.
• It can fail inspection twice. An active code keeps the light on, which fails the OBD check in states with emissions testing ^[3]. Some programs also test the cap itself. Washington, D.C. lists "gas cap leaks" and a missing or non-removable cap as typical reasons for failing its gas cap test ^[4].
• Your warning light is busy. While it is on for P0455, it cannot alert you to something new. If it ever starts flashing, read our guide to a flashing check engine light.

What causes P0455?

Causes are listed from most common and cheapest to least common. Every source we checked puts the gas cap first, but ALLDATA cautions that a loose or damaged cap is only one possible cause ^[1].

Cause	Typical clue	Who can check it
Gas cap missing, loose, or wrong part	Code appeared right after a fill-up	You
Gas cap seal torn or hardened	Cap turns but the rubber is cracked or flat	You
EVAP hose disconnected or split	Hose hanging loose, cracked fitting	You for visible hoses; a shop for the rest
Purge valve stuck open	Sometimes rough running right after refueling	Shop or scanner with active tests
Vent valve stuck open	No outside sign	Shop or scanner with active tests
Cracked charcoal canister	Damage near the tank, often after road debris	Shop
Leaking filler neck	Rust or damage near the fuel door	Shop

Carista lists the same core group: a loose, damaged, or missing cap, a cracked or disconnected EVAP hose, a failed purge or vent valve, and a cracked canister ^[5]. Identifix, a repair information provider, describes P0455 as the gross leak code, compared with about 0.040 inch for P0442 and about 0.020 inch for P0456 ^[6].

How do you diagnose P0455?

Start with free checks, and use a scanner before you spend money. Because a large leak is often something simple, most P0455 searches end at the gas cap or a hose you can see. If not, a smoke test finds the rest.

1. Scan first. Write down every stored and pending code and save the freeze frame. Other EVAP codes alongside P0455, such as a purge or vent circuit code, point the search toward that valve. If you have not used a scanner before, see our guide on how to use an OBD2 scanner.

2. Check the cap. Is it there? Did it click? Is the rubber seal whole? ALLDATA's first step is to make sure the cap is tightened properly and the seal is not cracked or damaged ^[1]. Replace a damaged cap with one made for your vehicle.

3. Look at the hoses you can reach. With the engine off and cool, trace the EVAP lines in the engine bay toward the purge valve. A hose that popped off is a common large leak. Do not work under a car that is supported only by a jack.

4. Drive and wait. The EVAP leak test is a periodic monitor. EPA guidance says these monitors run only after certain conditions are met, such as temperature and driving conditions ^[3]. Under OBD II the light goes off on its own after three consecutive trips without the fault ^[2].

5. Smoke test and valve tests. If the code comes back, a shop can fill the system with smoke to show the leak. Carista notes that the system is sealed at very low pressure, so a visual check alone often misses cracks ^[5]. A technician can also command the purge and vent valves with a scan tool to confirm they close.

How do you fix P0455, and what does it cost?

The repair matches the cause: a new gas cap, reconnecting or replacing a hose, a new purge or vent valve, or a canister or filler neck. We do not quote prices because we could not find a reliable, current U.S. source that covers these repairs. These factors set the bill:

• The part. A cap or a reconnected hose is a small job. A canister or vent valve mounted near the tank takes longer.
• Access. Parts under the car may need shields removed and the car raised on a lift.
• Diagnosis time. If the cap and visible hoses check out, expect to pay for a smoke test.
• Your inspection date. If you have a test coming up, fix the leak early so the monitors have time to run.

After the repair, the code can be cleared with a scanner. Clearing a code never repairs the leak; if the system is still open, P0455 returns after the next EVAP test. Clearing also resets the readiness monitors, and EPA guidance lists code clearing or a battery disconnection as reasons a car arrives at an inspection "not ready" ^[3]. Drive normally until the monitors complete.

How is P0455 different from P0442 and P0456?

All three come from the same EVAP leak test. P0455 means a large leak, P0442 a small one of about 0.040 inch, and P0456 a very small one of about 0.020 inch ^[6]. Large leaks are usually easier to find because the opening is bigger. If you are not sure what a second code on your scanner means, try our code lookup tool or browse the full trouble code list.