C054C – Vacuum Sensor A Circuit High (OBD2)
C054C means "Vacuum Sensor A 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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C054C quick summary
| Fault code | C054C |
|---|---|
| Meaning | Vacuum Sensor A Circuit High |
| 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.
C054C 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 C054C
- 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 C054C (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.Open circuit in the signal or ground wire
Most likelyA loose ground connection is a very common cause.
2.Signal wire shorted to the 5 V reference or battery voltage
3.Faulty brake booster pressure sensor
4.Vacuum leak in the booster hose or check valve
5.Weak vacuum pump
6.Wiring or connector fault
7.Brake booster diaphragm leak
Step-by-step diagnostic guide
- 1
Check the brake pedal
With the engine running, check that the brake pedal is soft and assisted.
- 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
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 C054C?
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 C054C?
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 C054C and similar codes?
C054C belongs to a group of codes for the same component that differ in the type or location of the fault: C0549 – Vacuum Sensor A Circuit Open; C054A – Vacuum Sensor A Range/Performance; C054B – Vacuum Sensor A Circuit Low; C054D – Vacuum Sensor B Circuit Open; C054E – Vacuum Sensor B Range/Performance; C054F – Vacuum Sensor B 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.