P0455: Evaporative Emission System Leak Detected (Large Leak)

What does P0455 mean?

The EVAP system collects fuel vapor from the tank and stores it in a charcoal canister until the engine can burn it. To test it, the ECM closes the system and checks whether it can build or hold pressure or vacuum.

P0455 means the system could not hold it at all, or lost it very quickly. This is consistent with a large opening, such as a missing gas cap, a disconnected hose or a valve stuck open.

The exact test strategy varies by manufacturer, and the monitor only runs under certain conditions, so a repair may not be confirmed straight away.

Common symptoms

• Check engine light on
• Fuel smell around the car in some cases
• Usually no change in driving
• Failed inspection where OBD codes and readiness are checked
• On some vehicles, difficulty refueling if a vent part is faulty

Common causes

The causes below are the ones commonly seen with P0455. The actual cause can vary by vehicle, engine and mileage, so treat this list as a starting point, not a diagnosis.

• A missing, loose or wrong gas cap
• A disconnected or cracked EVAP hose
• A purge valve or vent valve that is stuck open or not sealing
• A leaking charcoal canister or filler neck
• A faulty leak detection pump or tank pressure sensor on some vehicles

What to check with a scanner

• Check that the gas cap is present, the right type and tightened until it clicks.
• Look for disconnected hoses around the charcoal canister and purge valve, where visible and safe to reach.
• After a repair, clear the code and watch the EVAP monitor on the emissions readiness screen.
• If it returns, a smoke test is the usual next step.

How to read and clear P0455 with OBD Master Pro

1. Plug in your adapter. Connect a generic ELM327 (or ELM329) OBD2 adapter to the diagnostic port, usually under the dashboard on the driver's side. Bluetooth Classic, Bluetooth LE, Wi-Fi and USB OTG adapters are supported.
2. Switch the ignition on. Turn the key to ON or start the engine, depending on your car.
3. Connect in OBD Master Pro. Open the app, go to Connection and pick your adapter. Classic Bluetooth adapters connect from the Bluetooth tab, Bluetooth LE adapters from the BLE tab with no pairing needed. Use the generic OBD2 / EOBD profile or your car's brand profile.
4. Read the codes. Open the fault codes screen to see stored, pending and permanent codes. Confirm P0455 is listed and note any other codes, since they often explain the cause.
5. Check the details. Look at freeze frame data, Live data and, where your car supports it, Mode 06 test results (see the checks above).
6. Fix, then clear. After the repair, clear the codes to turn off the check engine light. Clearing also resets the emissions readiness monitors, so drive normally and check the readiness screen before any inspection. Permanent codes cannot be erased with a scan tool; they clear on their own once the car's self test passes.

Tip: you can also look up P0455 and other codes in the app's DTC search without connecting to a car. If your adapter will not connect, use Connection > Share connection log and email it to support. Many cheap ELM327 clones only handle basic engine OBD2, which is all you need for P0455.

When to see a mechanic

Get professional help if any of these apply:

• You smell raw fuel or see a leak (stop driving)
• The code returns after checking the gas cap
• You see other EVAP codes for purge or vent circuits
• You need an inspection soon

If you are unsure, a qualified technician can confirm the cause with tests such as smoke testing, pressure testing or scope readings. Clearing a code does not repair the fault.

Brand notes

Patterns below come from published manufacturer service information. They do not mean every vehicle of that brand has this problem, and a bulletin only applies to the vehicles it lists.

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