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C055B – ABS Pump Motor Supply Voltage Circuit A Low (OBD2)

C055B means "ABS Pump Motor Supply Voltage Circuit A Low". The voltage the control unit measures in the ABS hydraulic unit circuit is below the permitted minimum. For a sensor this usually means the signal wire is shorted to ground or the reference voltage is missing; for an actuator, that its control wire is shorted to ground.

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C055B quick summary

Fault codeC055B
MeaningABS Pump Motor Supply Voltage Circuit A Low
SystemChassis
VehiclesGeneric OBD2
SeverityHigh
Can I drive?Normal braking usually still works, but ABS and stability control may be off – drive carefully, keep more distance and avoid hard braking on wet or slippery roads. If the brake pedal feels soft or the red brake warning is on, stop and have the car checked.
Repair cost€0 – €1,500

How the ABS hydraulic unit works

The ABS hydraulic unit contains the valves and pump that modulate brake pressure at each wheel; these codes mean the control unit has detected a fault in a valve, the pump motor, its supply or the hydraulic circuit.

The ABS/ESP hydraulic unit sits between the brake master cylinder and the wheels and is bolted to the ABS control module. For each wheel it has an inlet valve that holds pressure and an outlet valve that releases it, plus a return pump driven by an electric motor. Traction control and stability control use additional switching valves to build pressure on their own, without the driver pressing the pedal.

The control module monitors the valve coils, the pump motor current and the supply voltage every time the car starts (you often hear a short buzz from the unit after moving off). Codes in this group cover individual inlet and outlet valves, a release phase that took too long, the pump motor and its supply, missing calibration after a module replacement and leaks in the brake hydraulic circuits.

With an ABS fault the normal brakes still work, but ABS, traction control and stability control are usually switched off. Common causes are a low battery voltage, a corroded main connector or ground on the ABS module, a seized pump motor after years without use, and internal valve or circuit board faults – the latter are often repaired by specialists rather than replaced.

C055B is a chassis code in the standard (SAE) numbering. Some manufacturers – for example GM, Opel and Ford on certain models – use their own meanings for some C codes, so always check the text your scan tool shows for your vehicle.

Symptoms of C055B

  • ABS, ESP and traction control warning lights on
  • Wheels lock under hard braking
  • Pump motor does not buzz after moving off, or runs continuously
  • Brake pedal feels different or pulses abnormally
  • Red brake warning light (hydraulic leak codes)

Most common causes of C055B (most likely first)

Mechanic's Pro Tip

Before buying a new ABS unit, check the battery and the module ground – and ask a module repair specialist. Many ABS control modules have known solder joint or pump relay faults that can be fixed for a fraction of the price of a new, coded unit.

  1. 1.Signal or control wire shorted to ground

    Most likely

    Look for chafed insulation where the harness touches the engine, exhaust brackets or body edges.

  2. 2.Missing 5 V reference or supply voltage

    An open reference wire or a shorted sensor elsewhere on the same reference circuit can pull it down.

  3. 3.Internal short in the ABS hydraulic unit

  4. 4.Low battery or charging voltage

    The pump and valves are checked against voltage limits – a weak battery is a classic cause of sporadic ABS codes.

  5. 5.Corroded main connector, fuse or ground on the ABS module

    The unit sits low in the engine bay and is exposed to water and road salt.

  6. 6.Seized or worn ABS pump motor

    Brushes and bearings wear; a motor that has not run for a long time can stick.

  7. 7.Brake fluid leak or air in the hydraulic unit

    Relevant for leak and release-time codes.

Step-by-step diagnostic guide

  1. 1

    Read codes and freeze-frame data

    Read the ABS/ESP module with a scan tool that can access it. Note the supply voltage stored with the code and any other modules reporting low voltage.

  2. 2

    Measure the signal voltage

    With the ignition on and the ABS hydraulic unit plugged in, back-probe the signal wire. Then unplug it and check the reference/supply and ground wires separately.

    Multimeter: Signal near 0 V = short to ground or no reference. Reference wire: about 5 V with ignition on. Signal wire to ground with connector unplugged: no continuity.

  3. 3

    Check battery, fuses and grounds

    Test the battery and charging voltage, then check the high-current ABS fuses and the module ground under load.

    Multimeter: Battery at rest about 12.4–12.7 V; engine running about 13.8–14.8 V; voltage drop on the ABS ground below about 0.1 V.

  4. 4

    Inspect the module connector

    Unplug the large connector on the ABS module and look for green corrosion, water and pushed-back pins.

  5. 5

    Run the actuator tests

    Use the scan tool to run the pump motor and the valve tests. A pump that does not run with good supply and ground points to the motor; a single valve failing its test points to the hydraulic unit or module.

  6. 6

    Repair, bleed and calibrate

    Repair wiring, have the module repaired or replace the unit. After hydraulic work, bleed the brakes with the scan tool's ABS bleeding routine and run any required calibration.

Frequently asked questions

Can I keep driving with C055B?

Normal braking usually still works, but ABS and stability control may be off – drive carefully, keep more distance and avoid hard braking on wet or slippery roads. If the brake pedal feels soft or the red brake warning is on, stop and have the car checked.

How much does it cost to fix C055B?

Typical repair costs are 0 € – 1500 €, depending on the cause and the vehicle. The most common fixes are: signal or control wire shorted to ground; missing 5 V reference or supply voltage; internal short in the ABS hydraulic unit. Always confirm the cause with the tests above before buying parts.

Do my brakes still work with an ABS fault?

Yes, the normal hydraulic brakes still work. Only the ABS function (and usually traction and stability control) is disabled, so the wheels can lock under hard braking.

Can an ABS module be repaired?

Often yes. Specialists repair the electronic part of many ABS modules; the hydraulic block is usually reused. After fitting, the module may need coding and the brakes bleeding.

What is the difference between C055B and similar codes?

C055B belongs to a group of codes for the same component that differ in the type or location of the fault: C053A – ABS Valves Missing Calibration; C053B – ABS Valves Supply Voltage Circuit Open; C055A – ABS Pump Motor Supply Voltage Circuit A High; C055C – ABS Pump Motor Supply Voltage Circuit A Range/Performance; C055D – Brake Hydraulic Circuit B Leak; C055E – Brake Hydraulic Circuit A Leak. The tests on this page apply to the whole group; the type of fault tells you where to start.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.