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C0550 – Vacuum Sensor B Circuit High (OBD2)

C0550 means "Vacuum Sensor B Circuit High". The voltage in the brake booster pressure sensor circuit is above the permitted maximum. For most sensors this points to an open signal or ground wire – the signal "floats" up to the reference voltage – or to a short to voltage.

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C0550 quick summary

Fault codeC0550
MeaningVacuum Sensor B Circuit High
SystemChassis
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.

C0550 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 C0550

  • 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 C0550 (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.Open circuit in the signal or ground wire

    Most likely

    A loose ground connection is a very common cause.

  2. 2.Signal wire shorted to the 5 V reference or battery voltage

  3. 3.Faulty brake booster pressure sensor

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

  5. 5.Weak vacuum pump

  6. 6.Wiring or connector fault

  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

    Check signal and ground

    With the ignition on and the brake booster pressure sensor plugged in, back-probe the signal and ground wires. A signal stuck at reference or battery voltage points to an open ground/signal wire or a short to voltage.

    Multimeter: Ground wire to battery negative: below about 0.1 V. Signal stuck at about 5 V (or 12 V) = open or short to voltage.

  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 C0550?

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 C0550?

Typical repair costs are 20 € – 400 €, depending on the cause and the vehicle. The most common fixes are: open circuit in the signal or ground wire; signal wire shorted to the 5 V reference or battery voltage; faulty brake booster pressure sensor. 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 C0550 and similar codes?

C0550 belongs to a group of codes for the same component that differ in the type or location of the fault: C054D – Vacuum Sensor B Circuit Open; C054E – Vacuum Sensor B Range/Performance; C054F – Vacuum Sensor B Circuit Low; C0561 – Vacuum Sensor A/B Correlation; C0566 – Brake Booster Solenoid Circuit Open; C0567 – Brake Booster Solenoid Circuit Low. 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.