What P0556 means
The brake booster pressure sensor (5 V reference, signal, ground) measures the vacuum in the brake booster.
How the engine computer detects it
On direct-injection, turbo, stop-start and hybrid engines intake vacuum is scarce, so the engine module watches it and creates vacuum when needed – closing the throttle, suppressing engine stop or running an electric vacuum pump. That makes it an input to idle and throttle control. P0556 means the signal is valid but implausible. GM compares it with intake vacuum (MAP) with the throttle nearly closed for over 3 s, filtered, type B with two trips. VW and Porsche also check the electric vacuum pump with it – Porsche compares the pressure at wake-up and 1.5 s later – so P0556 can also mean a weak pump or a leak. VW fixed false setting on the 2016–17 Jetta 1.4 T by software.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light and/or brake booster warning
- Stop-start not working
- Hard brake pedal with real vacuum loss
Common causes
- Leaking or kinked vacuum line or faulty check valve Most likely
- Sensor stuck or drifting Possible
- Electric vacuum pump or its relay weak (hybrid, Porsche, turbo DI) Possible
- Module software (false setting) Less likely
- Booster diaphragm leaking Less likely
How to diagnose P0556
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: brake booster pressure sensor, vacuum line, electric vacuum pump, engine control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Vehicles that report P0556 most
| Vehicle | Years | Reports | Share | Median miles |
|---|---|---|---|---|
| Chevrolet Malibu | 2016–2020 | 6 | — | |
| GMC Terrain | 2018–2020 | 5 | — |
Counts of owner complaints to the NHTSA that mention P0556 (15 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 5
- Vehicle speed, idle control and auxiliary inputs
- 56
- Specific fault
Questions drivers ask
Can I drive with code P0556?
Usually yes, carefully. Your hydraulic brakes normally keep working, but ABS, traction and stability control may be off, so the wheels can lock in a hard stop. If the red brake warning light is on, the pedal feels soft or different, or the car takes longer to stop, don't drive: have it towed.
Will P0556 clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
Can I fix P0556 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.
Often reported together
Codes that owners mention in the same NHTSA complaints as P0556. They can point to a shared cause, so it's worth checking them together.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.