C0591 indicates the dedicated brake booster control module has detected abnormally low current in the three-phase drive circuit (phases U, V, and W) that powers the electric brake booster motor. This usually means the motor is not drawing the expected current, which can reduce or eliminate power brake assist. The code is stored by the brake booster control module, not the engine computer.
The most common cause is an open motor winding, worn internal brushes, or a seized pump motor that cannot draw normal phase current. The motor is often integrated into the brake booster assembly and may need assembly replacement.
Corroded, chafed, backed-out, or high-resistance terminals in the phase U/V/W harness can cause low current flow. Intermittent opens or poor pin fit are common in the high-current motor feed circuit.
The module that switches current to the three-phase motor can fail internally, but this is less common than motor or wiring problems. It should be considered only after the motor and harness test good.
A weak battery, failing alternator, or excessive voltage drop can reduce available current and trigger a low-current fault even if the motor and wiring are otherwise normal.
A loose, corroded, or overloaded ground path can limit current delivery to the motor phases and produce the same low-current detection.
No. C0591 is set by the dedicated brake booster control module, not the PCM/ECM. Replacing the engine computer will not repair this fault and is the wrong diagnostic path.
It is not recommended. Power brake assist may be reduced or lost, requiring substantially higher pedal effort and increasing stopping distance.
On many applications the electric motor is integrated into the brake booster assembly and is not available separately, so the assembly is replaced. Always follow the specific service manual for the vehicle.
The module monitors total three-phase motor current. A single open winding, seized motor, or power/ground issue can cause the overall current detection to fall below the threshold, logging all phases together rather than indicating three separate failures.