C0099: 4WD/AWD Rear Differential Unit Actuator A Position Sensor A

What it means

This code identifies a fault associated with position sensor A for actuator A in the rear differential unit of a 4WD/AWD system. The control module sets it when the position signal is implausible or outside expected limits.

How it works

Position sensor A on rear differential actuator A reports the actuator’s position to the driveline control module. C0099 sets when this sensor’s signal is implausible or outside the expected limits, which can prevent the module from confirming actuator A’s position.

Common causes

Most likely first.

1. Faulty actuator position sensor
2. Damaged or corroded sensor connector
3. Open or shorted sensor wiring
4. Rear differential actuator fault

Symptoms

• AWD or 4WD warning
• AWD may be unavailable or limited
• Rear differential may not engage or disengage correctly

How to diagnose it

1. Read the freeze-frame and live data for actuator A position sensor A, and note whether its reported position changes when the actuator is commanded.
2. Inspect sensor A’s connector and wiring at rear differential actuator A for corrosion, loose terminals, damaged insulation, or a pulled wire.
3. Check whether the scan-tool position reading is stable and plausible through the available actuator commands, without forcing the driveline into an unsafe condition.
4. Using the vehicle’s wiring information, check the sensor circuit’s power, ground, and signal for opens, shorts, or poor connections.
5. If the circuit checks out, assess actuator A’s movement and sensor output using the manufacturer’s procedure to distinguish a sensor fault from a sticking actuator.
6. Clear the code and confirm that sensor A reports consistently during the operating conditions that originally set C0099.

Common fixes

• Clean and secure the position sensor A connector on actuator A.
• Repair damaged or corroded wiring in sensor A’s circuit.
• Replace position sensor A if its signal remains implausible with a verified circuit.
• Repair or replace rear differential actuator A if it cannot move through its expected range.
• Correct actuator calibration if the manufacturer’s procedure identifies it as the cause.

Can I keep driving?

The rear differential may not engage or disengage correctly, and AWD may be limited or unavailable. Avoid relying on AWD for traction until repaired; stop if the driveline binds, makes unusual noises, or behaves unpredictably.