P0556: Brake Booster Pressure Sensor Circuit Range/Performance
A generic powertrain (engine and transmission) trouble code, same meaning on every brand.
Test the pedal in the driveway first. Normal feel: drive gently to a shop - it may be just the sensor. Hard pedal or weak braking: stop, this code is then reporting a real assist failure.
Symptoms
- Brake warning light with check engine light
- Possible hard pedal
- Booster pump running constantly on some cars
- No symptom at all in the sensor-only case
Likely causes, most common first
- Booster pressure sensor drifted or failed
- Genuine vacuum loss in the booster circuit
- Leaking hose or check valve
- Wiring or connector fault at the sensor
Read P0556 yourself before paying for diagnosis
A basic Bluetooth OBD-II scanner ($20-40) plugs into the port under the dashboard and shows this code with live sensor data on your phone - useful both for confirming the fix and for talking to a shop from an informed position.
What is going on and how it gets diagnosed
P0556 flags implausible readings from the sensor that watches pressure inside the brake booster. Like every sensor rationality code it carries the same fork in the road: a lying sensor, or a truthful sensor reporting a real problem - and with brakes, you resolve that fork before driving, not after.
The test costs nothing: with the engine running, the pedal should feel like it always has. It also has a famous chapter - on some hybrid and direct-injection models this sensor and its booster assembly were the subject of large recalls, so running your VIN through the recall check on this site's page for your car takes a minute and may turn the whole repair free. The sensor alone is modest money; what it sometimes announces is not, which is exactly why the driveway pedal test comes first.
Repair options and cost
Typical US independent-shop ranges, parts and labor. Dealers run higher; regional labor rates vary.
Owner reports mentioning P0556
“I got off a highway exit ramp and with no warning my brakes were hard to press. There was no warning lights on my dash. The brakes take a lot of force to push for a stop. The warning light finally came on a day later with codes p0556 and u040171.”2020 GMC Terrain · Jun 2025
“The P0556 code gets stored when the PCM has detected a problem with the brake booster pressure sensor circuit because of abnormal feedback. Braking distance significantly affected by brake pedal becoming difficult to depress. Cause is the brake booster vacuum pump. Internet reports indicate multiple vehicle types during the late 20-teens and early 2020s. Problems are occurring during or shortly after warranty expiration. This is a very dangerous problem that could cause serious injury.”2020 Chevrolet Equinox · Feb 2023
“As I was driving in a 45 MPH two-lane road, the car in front of me wanted to turn left and when I stepped on the brake pedal, it was stiff. The car wouldn't slow down. So, I quickly stepped on the parking brake which helped and possibly saved my life and the life/lives of those I could have hit. One can only imagine the fear I had. The check engine light came on right after. When tested it reported P0556 - Brake Pressure Sensor; something that has to do with the Brake Booster Vacuum Pump. When I talked to a mechanic, he recommended I report this issue here because this should not happen.”2017 Chevrolet Malibu · Jun 2022
Show 1 more owner reports
“Tl* the contact owns a 2016 chevrolet malibu. While driving and depressing the brake pedal, the pedal traveled to the floorboard and there was a grinding noise. There was no warning indicator illuminated. In addition, the vehicle lost power and failed to accelerate. Keller chevrolet (located at 3600 genesee st, cheektowaga, ny 14225, (716) 650-4589) stated that the VIN was not included in a recall. The contact performed his own diagnostic testing and retrieved failure code: p0556 (brake booster pressure). The dealer and manufacturer were not notified of the failure. The approximate failure mileage was 32,000.”2016 Chevrolet Malibu · May 2019 · 32,000 mi
Cars that report P0556 most often
Based on mentions of this code in NHTSA owner complaints.
Frequently asked questions
Can I drive with P0556?
Test the pedal in the driveway first. Normal feel: drive gently to a shop - it may be just the sensor. Hard pedal or weak braking: stop, this code is then reporting a real assist failure.
How much does it cost to fix P0556?
Typical US repair costs range from $40 for the simplest fix (booster pressure sensor replacement) to $1,200 in the worst case. The right number depends on which cause your car has - see the causes list.
Related codes
The codes closest to P0556. They often appear together, and telling them apart is usually what narrows the diagnosis.
Definition per SAE J2012 standard nomenclature. Symptoms, causes and costs are editorial guidance - always confirm with a diagnosis of your specific vehicle.