C0584: Brake Booster Motor B Phase U-V-W Circuit Low

What it means

Brake Booster Motor B Phase U-V-W Circuit Low: the signal voltage is below the expected range. The electric brake booster generates the braking assist; a motor or sensor fault can reduce or disable power-assisted braking. Safety-relevant.

At a glance

• Usually lights the ABS or stability-control warning light
• The engine or gearbox may go into limp mode

How it works

The electric brake booster uses Motor B to provide braking assistance, and its U, V, and W phase circuits carry the motor drive signals. The control module sets C0584 when voltage in those phase circuits falls below the expected range, which may limit or disable brake assist.

Common causes

Most likely first.

1. Phase shorted to ground
2. Connector pins corroded - high contact resistance
3. Harness chafing through insulation

Symptoms

• Brake or ABS warning light
• Brake assist may be reduced or unavailable

How to diagnose it

1. Use a scan tool to record freeze-frame data and inspect brake-booster live data and related codes for the conditions when the low-voltage fault appeared.
2. With the vehicle safely parked, inspect Motor B's U, V, and W wiring for chafing or damaged insulation that could short a phase to ground.
3. Check the Motor B phase connector for corrosion, moisture, loose terminals, or poor pin contact.
4. Using the wiring information for the vehicle, test the phase circuits for a short to ground and check continuity with power isolated as specified by the manufacturer.
5. If wiring and connections pass, test the booster motor and control module circuits using the manufacturer's diagnostic procedure.

Common fixes

• Repair chafed U, V, or W phase wiring that is shorting to ground.
• Clean or repair corroded or loose Motor B connector terminals.
• Repair or replace damaged sections of the phase harness.
• Replace the brake booster motor or control module only if testing identifies it as faulty.

Can I keep driving?

Because brake assist may be reduced or unavailable, avoid driving if the brake pedal feels unusually hard or braking response changes. Arrange safe transport for diagnosis; continued driving with impaired assist can make stopping harder.