Plug-in and some full hybrids keep the fuel tank sealed while driving, separated from the charcoal canister by a valve. When the driver presses the fuel button, the system opens the valve, lets the pressure escape into the canister and only then unlocks the filler door. A switch reports whether the door is open or closed, so that the tank can be sealed again afterwards. P04B8 is the general circuit fault or open circuit of the door switch: the module cannot read the door state. If the door state is unknown, the system leaves the tank open to the canister as a precaution and suspends the leak tests. No manufacturer document available to us uses this code; the description follows CARB's input component rules and patents for the door switch.
Low severity. Can I keep driving?You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.
Symptoms
- Check engine light on
- Fuel door open message although the door is closed, or missing when it is open
- Door may not open or only after a delay
- EVAP readiness not set
Most likely causes
- Wire or connector at the door switch open (side panel, moisture)
- Switch or sensor faulty internally
- Ground or supply interrupted
- Module input faulty
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: easy
- It affects emissions.
What each character of P04B8 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 4: Emissions
- B8: Specific fault in that system
Frequently asked questions
Can I keep driving with code P04B8?
You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.
What causes code P04B8?
The most common causes are: wire or connector at the door switch open (side panel, moisture); switch or sensor faulty internally; ground or supply interrupted.
Related codes
- P04B9Fuel Fill Door Position Sensor/Switch Circuit Low
- P04BAFuel Fill Door Position Sensor/Switch Circuit High
- P04B7Fuel Fill Door Position Sensor/Switch Circuit Intermittent/Erratic
- P04DCFuel Fill Door Position Sensor/Switch Circuit Range/Performance
- P04B5Fuel Fill Door Stuck Open
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.