What P0557 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. P0557 means the signal voltage is too low: GM 2018 below 5 % of the 5 V reference in 320 of 400 samples (about 5 s), GM 2006 below 0.04 V; type B, two trips. Typical are a signal shorted to ground or an open signal or 5 V line. As low voltage means plenty of vacuum, the module would wrongly assume enough – so it uses a substitute value.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light or brake booster warning
- Stop-start inactive
- Often no change in driving
Common causes
- Signal line open or shorted to ground, connector loose or corroded Most likely
- 5 V reference open or shorted to ground Possible
- Sensor faulty internally Possible
- Engine module Less likely
How to diagnose P0557
- 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, sensor wiring, 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.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 5
- Vehicle speed, idle control and auxiliary inputs
- 57
- Specific fault
Questions drivers ask
Can I drive with code P0557?
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 P0557 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 P0557 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.
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.