C054F – Vacuum Sensor B Circuit Low (OBD2)
C054F means "Vacuum Sensor B Circuit Low". The voltage the control unit measures in the brake booster pressure sensor 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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C054F quick summary
| Fault code | C054F |
|---|---|
| Meaning | Vacuum Sensor B Circuit Low |
| System | Chassis |
| Vehicles | Generic OBD2 |
| Severity | Medium |
| 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.
C054F 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 C054F
- 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 C054F (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.Signal or control wire shorted to ground
Most likelyLook for chafed insulation where the harness touches the engine, exhaust brackets or body edges.
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.Internal short in the brake booster pressure sensor
4.Faulty brake booster pressure sensor
5.Vacuum leak in the booster hose or check valve
6.Weak vacuum pump
7.Wiring or connector fault
Step-by-step diagnostic guide
- 1
Check the brake pedal
With the engine running, check that the brake pedal is soft and assisted.
- 2
Measure the signal voltage
With the ignition on and the brake booster pressure sensor 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
Read codes and booster vacuum
Read all codes and watch the booster pressure in live data.
- 4
Check hoses and check valve
Check the vacuum hose and one-way valve for leaks.
- 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
Replace and verify
Replace the sensor or fix the vacuum leak and check the brakes.
Frequently asked questions
Can I keep driving with C054F?
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 C054F?
Typical repair costs are 20 € – 400 €, 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 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 C054F and similar codes?
C054F belongs to a group of codes for the same component that differ in the type or location of the fault: C054C – Vacuum Sensor A Circuit High; C054D – Vacuum Sensor B Circuit Open; C054E – Vacuum Sensor B Range/Performance; C0550 – Vacuum Sensor B Circuit High; C0561 – Vacuum Sensor A/B Correlation; C0566 – Brake Booster Solenoid Circuit Open. 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.