P04B9Fuel Fill Door Position Sensor/Switch Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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. P04B9 means the door switch signal is below its valid range – the signal line is shorted to ground or the switch is shorted internally. Stellantis checks its status line for a short to ground under P04B5, so depending on the design such a short can also appear as an open door. 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.
- What you may notice
- 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
- Likely causes
- LikelySignal line shorted to ground (chafed or pinched)
- PossibleSwitch shorted internally
- PossibleWater or corrosion in the connector
- RareModule input faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €30 – €250
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.