P0440: Evaporative Emission System

What does P0440 mean?

The EVAP system stores fuel vapor from the tank in a charcoal canister and later draws it into the engine to be burned. The purge valve controls that flow and the vent valve lets fresh air into the canister.

P0440 is a general EVAP fault. It can be set by a leak or by a component that does not work as expected during the system self test.

Common symptoms

• Check engine light on
• Usually no change in how the car drives
• Possible fuel vapor smell
• Possible failed emissions inspection

Common causes

The causes below are the ones commonly seen with P0440. 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 damaged fuel cap
• A faulty purge valve
• A damaged charcoal canister
• Cracked or disconnected EVAP hoses

What to check with a scanner

• Check the fuel cap first. Make sure it seals and clicks.
• Inspect visible EVAP hoses near the engine and the canister for cracks.
• After a repair, clear the code and watch the readiness screen until the EVAP monitor runs again.
• A smoke test is the usual way to find leaks that are not visible.

How to read and clear P0440 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 P0440 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 P0440 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 P0440.

When to see a mechanic

Get professional help if any of these apply:

• The code returns after checking the fuel cap
• You smell strong fuel vapor
• Several EVAP codes are stored together

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.

Related trouble codes

See all OBD2 trouble code guides

Related guides

Check engine light on: what to do · How to read an OBD2 code (P, B, C, U) · Stored, pending and permanent codes and freeze frame · OBD2 readiness monitors explained · OBD2 glossary · OBD2 by car brand