OBD-II reference

All 9.533 generic OBD-II codes, explained in detail.

Causes, inspection steps and sources — a comprehensive DIY archive. Created and maintained with love for the detail. Good luck and have fun!

Powertrain generic / SAE

P21F9

Fuel Control System B Too Rich at Idle (Bank 1)

The secondary fuel control loop B on bank 1 reports a rich condition outside idle.

SystemPowertrain
Typegeneric / SAE
GroupP2
Range / number1 / F9

Possible causes

  • Injectors leak at low duty cycle high

Symptoms

  • Check engine light on
  • Increased fuel consumption
  • Black smoke from exhaust on acceleration
  • Smell of unburned fuel, fouled spark plugs

Inspection / repair

  • Difficulty: medium
  • DIY: possible
  • Estimated cost: 80–1000 €
  • Estimated time: 0.7–4 h
  • Component: fuel injector
  • Component: fuel pressure regulator
  • MIL / check engine light
  • emissions-relevant

Sources

References

How to read OBD-II codes

The first letter names the vehicle system, the second digit distinguishes generic from manufacturer-specific, the remaining digits group fault area and number.

PPowertrain: engine, transmission, emissions
BBody: chassis trim, comfort, interior
CChassis: suspension, brakes, steering
UNetwork: CAN, control modules, communication