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C0585 — Brake Booster Motor B Phase U V W Circuit High

Generic OBD-II · all makesChassis

This code indicates that the control module has detected an abnormally high voltage in the Phase U, V, or W circuits of the Brake Booster Motor B. This typically points to a short to power, a failed internal motor winding, or a ground reference issue within the electronic brake booster assembly.

Symptoms

What causes it — most likely first

1. Brake Booster Motor Failure

Internal shorting or an open winding within the brake booster motor can cause abnormal voltage readings on the phase circuits.

2. Wiring or Connector Damage

A short to voltage, corroded terminals, or a poor ground connection in the phase U/V/W circuits can trigger a high voltage condition.

3. Brake Booster Relay Failure

A stuck closed relay can supply continuous battery voltage to the motor circuit, causing a high voltage state.

4. Brake Booster Control Module Failure

The internal driver circuitry of the dedicated brake control module may fail, incorrectly applying or reading high voltage on the phase circuits.

5. PCM/ECM Failure

The main powertrain control module is rarely the cause, but could theoretically fail to interpret the high voltage signal correctly.

How it's diagnosed

  1. Scan for all ABS and chassis codes to identify related system failures.
  2. Visually inspect the wiring harness and connectors between the brake booster motor and its control module for damage, corrosion, or shorting.
  3. Measure the resistance of the Phase U, V, and W motor windings to check for internal shorts or opens.
  4. Check the brake booster relay and power supply circuits for stuck contacts or continuous power.
  5. Verify all ground connections for the brake booster system are clean and secure.
  6. Test the brake booster control module output signals if the motor and wiring test within specifications.

Code facts & related codes

SystemChassis
StandardizationSAE generic — identical on every make

Frequently asked

Does C0585 mean my engine computer (PCM) is bad?

No, replacing the PCM is a common misdiagnosis for this code. The fault is almost always in the brake booster motor, its wiring, or the dedicated brake control module, not the main powertrain computer.

Is it safe to drive with this code?

No, a high voltage condition in the brake booster motor circuit can result in a sudden loss of braking assist, making the pedal very hard to push and significantly increasing stopping distance.

What does 'Phase U V W Circuit High' mean?

It means the system detected higher than expected voltage in the three-phase motor circuit that drives the brake booster, usually indicating a short to power, a poor ground, or an internal motor failure.

Can I just replace the brake booster motor to fix this?

Not without proper diagnosis. If a wiring short or a failed relay caused the code, installing a new motor will not fix the issue and could potentially damage the replacement component.