Powertrain, generic · Auxiliary emission controls

P04B9 Fuel Fill Door Position Sensor/Switch Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

Signal line shorted to ground (chafed or pinched)
Switch shorted internally
Water or corrosion in the connector
Module input faulty

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

Repair estimate

DifficultyEasyDIY possible
Parts and labour€30–250
Time0.5–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

fuel fill door position switchswitch wiringengine control module

Related codes

P04B8P04BAP04B7P04DCP04B5

References: law.cornell.edu, patents.google.com, patents.google.com.