C1851 The AWD control unit found a fault in the rear differential pump motor circuit

What does C1851 mean on a Honda?

C1851. The AWD control unit found a fault in the rear differential pump motor circuit. The rear differential may not engage and the warning light can come on. This Honda-enhanced code applies to Honda and Acura models that use this code.

Common causes of C1851

• Faulty rear differential pump motor
• Rear differential pump motor harness is open or shorted
• Rear differential pump motor circuit poor electrical connection
• Faulty AWD control unit
• Water or debris contamination in the rear differential

How to fix C1851

1. Confirm the code with a scan tool and check the AWD warning light
2. Inspect the pump motor connector and wiring for corrosion, moisture, and loose pins
3. Test the pump motor circuit and operation against the AWD control unit command
4. Check the wiring and differential fluid before replacing the motor
5. Replace the rear differential pump motor if it fails testing, then clear codes and retest

Sources and review

• Last updated 2026-09-29.