What it means
Brake Booster Motor A Phase U-V-W Current Low: the control module has detected this fault condition. 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
Brake booster motor A supplies power-assist force, and its U, V, and W phase circuits carry the current needed to run it. The control module monitors that current; C0591 sets when motor A phase current is lower than expected, often because a phase connection is open or resistive.
Common causes
Most likely first.
- Open or high resistance in motor A phase wiring
- Poor motor connector contact
- Motor winding fault
- Control module fault
Symptoms
- ABS warning light on
- ESC/stability control deactivated
- Possible reduced brake assist
- Speedometer may behave erratically
How to diagnose it
- Record freeze-frame data and related booster codes to determine whether low current was detected during a motor A command.
- Inspect motor A phase connectors and wiring for loose or corroded terminals, broken conductors, or other damage.
- Check terminal fit and contact condition at the motor A connections, since a poor contact can restrict phase current.
- Test continuity and resistance in each motor A U, V, and W phase path and compare results with the manufacturer's specifications.
- If the phase wiring passes, test motor A windings and the drive circuit using manufacturer procedures before considering a module fault.
Common fixes
- Restore a loose or corroded motor A phase connector contact.
- Repair an open or high-resistance section of motor A phase wiring.
- Replace motor A if its winding checks fail the manufacturer's specifications.
- Repair or replace the motor drive circuit or control module only after the motor and wiring pass testing.
Can I keep driving?
Brake assist may be reduced or unavailable if motor A cannot draw the current needed to operate. Avoid driving with a brake warning or increased pedal effort, and have the fault diagnosed promptly.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.