P0556: Brake Booster Pressure Sensor Circuit Range/Performance (Meaning, Causes, Fixes)
What P0556 means
The power brake booster uses engine vacuum to make the brake pedal easier to push. The brake booster pressure sensor measures how much vacuum is in the booster and reports it to the engine computer (PCM). RepairPal says P0556 sets when the PCM sees an implausible signal from this sensor; OBD-Codes.com says it sets when the sensor circuit is out of range or not performing properly. OBD-Codes.com lists makes with this sensor including Fiat, Alfa Romeo, Nissan, Ford, Dodge, Pontiac, Chevrolet, Acura, Subaru and Hyundai, and notes the brake warning light may come on or flash when braking. The two sources differ on severity (RepairPal 'Moderate', OBD-Codes.com says it can become a safety issue) and on how many causes they list: OBD-Codes.com adds vacuum lines and the brake booster itself, which RepairPal doesn't mention. Related booster sensor codes are P0555 (circuit), P0557 (circuit low), P0558 (circuit high) and P0559 (intermittent).
Severity: can I keep driving?
Drive with caution, fix soon
Get it checked soon, and sooner if the brake pedal feels hard. RepairPal rates P0556 'Moderate'. OBD-Codes.com says severity depends on the specific fault and can progress until the vehicle's safety becomes an issue requiring immediate attention. If the brake pedal takes noticeably more effort to push or the car doesn't stop normally, don't drive it until the brakes are checked.
Common symptoms
- Check engine (service engine soon) light on (listed by OBD-Codes.com)
- Brake warning light flashes when braking, or stays on (listed by OBD-Codes.com)
- More effort needed to push the brake pedal (listed by OBD-Codes.com)
- RepairPal lists no specific symptoms for this code
Likely causes
Roughly most to least common, based on the sources below. Your vehicle may differ.
- Faulty brake booster pressure sensor (both sources)
- Damaged or faulty wiring (both sources); OBD-Codes.com also lists a corroded or damaged connector
- Worn, cracked or leaking vacuum lines (listed by OBD-Codes.com)
- Faulty brake booster (listed by OBD-Codes.com)
- Faulty PCM (both sources)
Diagnostic steps you can do
Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.
- Check for technical service bulletins (TSBs) for your vehicle first; OBD-Codes.com says this is the first troubleshooting step.
- Find the brake booster, its pressure sensor and the vacuum line to the booster.
- Inspect the vacuum lines for cracks, leaks, loose fittings or weather damage.
- Inspect the sensor wiring and connector for rubbing, bare wires, burn spots, corrosion or damaged pins.
- With a multimeter and your vehicle's service data, check the sensor's power, ground and signal. OBD-Codes.com says correct voltage going into the sensor with no output usually means the sensor has failed internally. Voltage specs vary by vehicle.
- Clear the code and see if it returns before buying parts.
Typical fixes
- Replace leaking or cracked vacuum lines to the brake booster
- Clean corrosion from the sensor connector
- Repair or replace damaged wiring
- Replace a faulty brake booster pressure sensor
- Replace the brake booster if it's faulty (OBD-Codes.com)
- Flash (reprogram) or replace the PCM, only after everything else checks out
Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.
🛠 When to call a pro
- The brake pedal is hard to push or the car doesn't stop normally
- The brake warning light stays on
- You don't have service data for the sensor's voltage specs, which OBD-Codes.com says vary by year, make and model
- The code keeps coming back after a new sensor
Where to buy
Plain search links. Always match parts to your exact year, make, model and engine.
FAQs
Is it safe to drive with P0556?
RepairPal rates it 'Moderate', but OBD-Codes.com warns it can become a safety issue because it involves the power brakes. If the pedal is hard to push or the brake light stays on, get the brakes checked before driving.
Does P0556 mean I need a new brake booster?
Not necessarily. Both sources list the sensor, wiring and PCM as causes; only OBD-Codes.com lists the booster itself, along with vacuum lines. Check the vacuum lines, wiring and sensor first.
What's the difference between P0556 and P0557?
Both are brake booster pressure sensor codes. P0556 means the signal is out of range or implausible; P0557 means the circuit voltage is too low.
How much does it cost to fix P0556?
Neither source gives a repair cost. It depends on whether it's a vacuum line, wiring, the sensor or the booster, so get a diagnosis and a written estimate first.
Sources
Last reviewed 2026-10-23. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.