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DTC codes / C05xx · Brakes, steering and suspension / C056A

C056ABrake Booster Pressure Sensor Power Supply Circuit

C056A indicates that the brake booster pressure sensor's power supply circuit has a fault, meaning the anti-lock brake system (ABS) control module is not receiving the expected voltage on the sensor's feed wire. The module monitors this circuit and sets the code when the voltage falls outside the normal operating range, which can prevent the sensor from providing accurate brake booster pressure data.

SystemChassis
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaBrakes, steering and suspension

Symptoms you may notice

  • Illumination of the ABS warning light, and often the traction control and stability control lights as well
  • A hard brake pedal with increased effort required to stop the vehicle
  • Reduced or disabled power brake assist, especially noticeable at low speeds
  • The ABS system may be disabled, though base braking usually remains functional
  • Often no symptoms besides the warning light, particularly if the fault is intermittent

Common causes, most likely first

  1. Open or high-resistance wire in the sensor's power supply circuit, such as a broken conductor or corroded connector
  2. Short to ground or short to voltage on the power supply wire
  3. Failed brake booster pressure sensor with an internal fault affecting its power terminal
  4. Corroded, loose, or damaged electrical connector at the sensor or ABS module
  5. Blown fuse or faulty relay that supplies power to the sensor circuit
  6. Faulty ABS control module not supplying the correct reference voltage

How to diagnose and fix it

  1. Connect a scan tool and record all stored codes and freeze-frame data, then clear codes and verify whether C056A returns
  2. Visually inspect the brake booster pressure sensor connector and the ABS module connector for corrosion, loose pins, or damaged wiring
  3. Check the fuse and relay that feed the sensor circuit, replacing any that are blown or faulty
  4. Use a digital multimeter to measure voltage on the sensor's power supply wire with the key on, comparing to the manufacturer's specification
  5. Perform a voltage drop test on the power supply circuit to identify excessive resistance under load
  6. If voltage is missing or out of range, trace the wire from the ABS module to the sensor and repair any open or short circuits
  7. If the circuit tests good, substitute a known-good sensor or perform a sensor signal test to confirm whether the sensor itself is faulty

What the power supply code really tells you

  • A code for the power supply circuit does not automatically mean the sensor has failed; the fault often lies in the wiring or connectors, so testing the circuit before replacing parts saves time and money.
  • Intermittent C056A codes are frequently caused by a chafed wire that only shorts when the engine moves or the vehicle hits a bump, so wiggle-testing the harness while monitoring live data is essential.
  • Some vehicles use a shared reference voltage for multiple sensors, and a short in one sensor's circuit can pull down the voltage for others, setting additional codes that may distract from the real problem.
  • The ABS module may set this code if its internal power supply to the sensor fails, meaning the module itself can be the culprit even when all external wiring tests perfectly.
  • A weak or failing battery or charging system can cause low voltage conditions that trigger this code intermittently, so always verify battery and charging system health before condemning the sensor circuit.

Can I keep driving with C056A?

The vehicle is generally safe to drive because base braking remains functional, but you should avoid hard braking or emergency maneuvers since ABS and stability control may be disabled. Have the fault diagnosed soon, as a loss of power brake assist can increase stopping distances.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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