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P0556 – Brake Booster Pressure Sensor Circuit Range/Performance (OBD2)

P0556 means "Brake Booster Pressure Sensor Circuit Range/Performance". The brake booster pressure sensor signal is electrically present but implausible: it does not match what the control unit expects for the current operating conditions or from other sensors. This is a performance fault, not a broken wire.

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P0556 quick summary

Fault codeP0556
MeaningBrake Booster Pressure Sensor Circuit Range/Performance
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityMedium
Can I drive?Yes, if the brake pedal feels normal. If the pedal is hard and braking needs much more force, stop and have the car checked.
Repair cost€20 – €400

How the brake booster pressure sensor works

The brake booster pressure sensor measures the vacuum in the brake servo, so the engine control unit can ensure there is enough vacuum for braking assistance.

The brake booster uses engine vacuum (or a vacuum pump on diesels and some petrol engines) to multiply the force on the brake pedal. A pressure sensor on the booster tells the control unit how much vacuum is available. If it is too low, the control unit can change throttle or engine strategy, run an electric vacuum pump or prevent the start-stop system from switching off the engine.

Codes P0555–P0559 report circuit, range and intermittent faults of this sensor.

A faulty sensor can disable start-stop or cause warnings, but a real lack of vacuum makes the brake pedal hard – which is a safety issue.

On VAG vehicles (VW, Audi, Škoda, Seat, Cupra), VCDS and older VAG testers can show this code as the five-digit VAG code 16940 (P0556). Both numbers mean the same fault.

Symptoms of P0556

  • Check Engine or brake warning
  • Start-stop system not working
  • Hard brake pedal (real vacuum problem)
  • Hissing near the brake booster
  • Often no other symptoms

Most common causes of P0556 (most likely first)

Mechanic's Pro Tip

If start-stop no longer works, the brake booster sensor is one of the less obvious reasons – check its value in live data.

  1. 1.Brake booster pressure sensor worn, contaminated or out of calibration

    Most likely

    The signal drifts slowly, so it is still within the electrical limits but wrong.

  2. 2.Mechanical problem in the system the brake booster pressure sensor measures or controls

    For example a leak, a sticking part or a restriction that really changes the measured value.

  3. 3.High resistance in a connector or ground

    Corrosion causes a voltage drop that shifts the reading.

  4. 4.Faulty brake booster pressure sensor

  5. 5.Vacuum leak in the booster hose or check valve

  6. 6.Weak vacuum pump

  7. 7.Brake booster diaphragm leak

Step-by-step diagnostic guide

  1. 1

    Check the brake pedal

    With the engine running, check that the brake pedal is soft and assisted.

  2. 2

    Compare live data with expected values

    Read the brake booster pressure sensor values in live data and compare them with related readings (engine speed, load, temperature or pressure) and with the specification. A value that is electrically fine but wrong for the conditions confirms a performance fault – then check the mechanical side before replacing the part.

  3. 3

    Read codes and booster vacuum

    Read all codes and watch the booster pressure in live data.

  4. 4

    Check hoses and check valve

    Check the vacuum hose and one-way valve for leaks.

  5. 5

    Measure vacuum

    Measure the vacuum with a gauge and compare with the sensor reading.

    Multimeter: Typical booster vacuum: about 0.6–0.8 bar below atmospheric at idle.

  6. 6

    Replace and verify

    Replace the sensor or fix the vacuum leak and check the brakes.

Frequently asked questions

Can I keep driving with P0556?

Yes, if the brake pedal feels normal. If the pedal is hard and braking needs much more force, stop and have the car checked.

How much does it cost to fix P0556?

Typical repair costs are 20 € – 400 €, depending on the cause and the vehicle. The most common fixes are: brake booster pressure sensor worn, contaminated or out of calibration; mechanical problem in the system the brake booster pressure sensor measures or controls; high resistance in a connector or ground. Always confirm the cause with the tests above before buying parts.

Is a brake booster sensor code dangerous?

The sensor fault itself is not, but a real vacuum loss makes braking harder – check the pedal feel.

Why does it affect start-stop?

The engine must keep running if there is not enough vacuum for the brakes.

What is the difference between P0556 and similar codes?

P0556 belongs to a group of codes for the same component that differ in the type or location of the fault: C0568 – Brake Booster Solenoid Circuit High; C0569 – Brake Booster Solenoid Performance; P0555 – Brake Booster Pressure Sensor Circuit; P0557 – Brake Booster Pressure Sensor Circuit Low Input; P0558 – Brake Booster Pressure Sensor Circuit High Input; P0559 – Brake Booster Pressure Sensor Circuit Intermittent. 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.