C1256

Symptoms

• Multiple dash warning lights (brake, ABS) often with no immediate driveability symptom
• a persistent buzzer that comes and goes
• brake accumulator pump running very frequently
• in fail-safe mode the brake pedal becomes very hard to press and stopping distances increase significantly (loss of power braking, ABS and stability control).

Probable causes

• On 2004-2011 Prius the code almost always means the brake actuator assembly has failed due to a slow internal leak: the pump runs more and more frequently to maintain accumulator pressure until it can no longer keep up. Other possible causes per sources: malfunctioning ABS pump/motor
• low brake fluid
• or a leak in the brake system.

Diagnostic steps

1. Confirm the make — on Toyota/Lexus proceed with the accumulator diagnosis; on GM the code points to the EBCM instead.
2. Check brake fluid level and inspect for external leaks.
3. Listen for the accumulator pump run frequency — a pump cycling roughly every 15 seconds indicates the actuator is close to failure (per go-parts.com).
4. Perform the pressure-hold test: a voltage drop of more than 0.20 V in 30 seconds on the pressure sensor signal indicates a definitive internal leak in the actuator assembly (per go-parts.com).
5. If the internal leak is confirmed, replace the brake actuator assembly — sources recommend a new OEM unit, as used or remanufactured units have a high failure rate.
6. Warn the customer: while the code is active the system is in fail-safe with degraded braking; continued driving is dangerous.