C0100: Rear ABS (Both Wheels) Solenoid/Motor #1 Circuit Malfunction

What it means

This code identifies a fault in circuit 1 for the rear ABS solenoid or motor serving both rear wheels. The ABS control module sets it when its circuit checks detect an electrical fault such as an open or short.

How it works

The ABS hydraulic control unit uses solenoid or motor circuit 1 to control braking pressure for both rear wheels. The ABS module monitors that shared rear circuit for an electrical fault such as an open or short. C0100 can disable ABS or traction control, leaving the rear wheels more likely to lock during hard braking.

Common causes

Most likely first.

1. Faulty ABS hydraulic control unit solenoid
2. Open or shorted solenoid wiring
3. Loose or corroded hydraulic unit connector
4. ABS control module fault

Symptoms

• ABS warning light
• Traction control warning light
• ABS or traction control may be disabled
• Wheel lockup may occur during hard braking

How to diagnose it

1. Scan the ABS module and record freeze-frame or event data, if available, to determine when circuit 1 faulted and whether other hydraulic unit or supply codes are present.
2. Inspect the hydraulic control unit connector and the wiring for circuit 1 for corrosion, loose pins, or damage before replacing any ABS component.
3. Check the ABS module power and ground connections and relevant fuses using the vehicle's service information.
4. Use the wiring diagram to test circuit 1 continuity and check for shorts, isolating the harness from the hydraulic unit and module as specified by the manufacturer.
5. If wiring and supply checks pass, carry out the manufacturer's hydraulic unit solenoid or motor circuit test to distinguish a unit fault from an ABS module driver fault.

Common fixes

• Clean and secure the ABS hydraulic unit connector.
• Repair open, shorted, or damaged wiring in rear ABS circuit 1.
• Replace a failed fuse or repair a verified ABS power or ground connection.
• Replace the hydraulic control unit if its circuit 1 solenoid or motor fails testing.
• Replace the ABS control module only if its driver is confirmed faulty.

Can I keep driving?

With the ABS or traction control warning on, those functions may be disabled and the rear wheels can lock during hard braking; leave extra stopping distance and avoid abrupt braking where possible. If the brake pedal or normal braking changes, stop safely and arrange prompt inspection.