Diagnostic article Generic OBD-II
P0557: Low brake-booster pressure sensor voltage
P0557 identifies an electrically low booster-pressure sensor signal in a specified GM implementation.
Read before applying this article
Documented application
2009 General Motors diagnostic parameter file 09OBDG02 Engine Diagnostics.pdf. Section-specific modules and calibrations; no per-code model or engine applicability established.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
The P0557 monitor in GM's group 02 table checks brake-booster pressure sensor voltage. Its published low threshold is below 0.195 volts, with a two-second detection interval. This is an electrical signal boundary for the documented application, not a direct statement about the actual booster pressure.
Low voltage is not a complete brake diagnosis
A low reported signal can make pressure information unreliable. The code does not measure braking performance or establish whether assistance is adequate. If pedal effort or braking behavior changes, assessment must follow the vehicle's brake-system guidance and appropriate professional procedures.
The neighboring P0558 entry checks the opposite electrical direction, while P0556 evaluates pressure plausibility. Separating a low circuit signal from an in-range but implausible pressure reading prevents the wrong diagnostic path from being chosen.
Establish applicability before electrical testing
The source explicitly restricts these rows to internal application groups. It does not supply a model-wide fitment list or a universal connector pinout. Use the exact vehicle wiring and service information rather than applying the printed threshold to any car carrying P0557.
Preserve the low-signal record and related faults before work. Verification should demonstrate a valid sensor signal, completed circuit monitoring and any brake-function checks required by the service procedure. An erased code, by itself, does not establish that either the signal or brake assistance has been restored.
Source and applicability
2009 General Motors diagnostic parameter file 09OBDG02 Engine Diagnostics.pdf. Section-specific modules and calibrations; no per-code model or engine applicability established. This explanation describes the documented monitor, not a universal repair procedure.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Brake Booster Pressure Sensor Circuit Low
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT