Chassis, generic

C009E 4WD/AWD Rear Differential Unit Actuator B Position Sensor B

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

Middle channel B of the second rear-unit actuator is reported here. It belongs to designs with two independently controlled elements; GKN's Twinster, used on the Focus RS rear drive module, can send torque to either rear wheel separately through two wet clutches. A fault in channel B of actuator B is typically an interrupted, grounded or frozen signal while channels A and C still switch. The detection logic in a Hyundai Kefico patent fits this case: it takes a healthy channel as reference, and a partner that never changes while the reference toggles is stored as defective. That patent also lets the motor run on in an emergency mode from the remaining edges. No public manufacturer document names C009E. The usual result is that the controller stops using the affected side and shows an AWD warning.

Likely causes

Channel B signal of actuator B interrupted or grounded
Harness chafed near the rear axle or exhaust heat shield
Internal sensor failure in actuator B

Symptoms

  • AWD fault message, one rear wheel no longer driven actively
  • Drift or sport drive modes unavailable

Repair estimate

DifficultyHardWorkshop job
Parts and labour€80–1,200
Time0.5–3 h

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

Parts involved

rear diff actuator b pos b

Related codes

C009DC009FC00A0C009A

References: just-auto.com, patents.google.com, mobilityengineeringtech.com.