P0558: High booster-pressure sensor voltage needs electrical interpretation

P0558 identifies the high-voltage direction of the brake-booster pressure sensor circuit check in GM's group 02 summary. The documented threshold is above 4.85 volts. It is a sensor-signal criterion, not a direct measurement proving that booster pressure is physically excessive.

Separate the reported signal from brake assistance

The neighboring P0556 monitor evaluates whether pressure information is plausible relative to other pressures. P0558 instead concerns an electrically high signal. Resolving electrical credibility comes before using that reported pressure to draw conclusions about the vacuum system.

Neither code alone establishes how much brake assistance is available. Changed pedal effort or braking behavior requires appropriate assessment under the vehicle's brake guidance; a generic code interpretation cannot provide a safe-driving determination.

Use the specified application and circuit procedure

The source restricts the booster-sensor entries to particular internal application groups. That does not establish fitment across every model covered by the overall GM document. The exact vehicle wiring and sensor specification are needed before applying a test value or choosing a component.

Retain the high-signal evidence and companion faults, then follow the prescribed circuit diagnosis. The summary does not identify a connector pin or prove which component caused the voltage. After correction, verify valid pressure information, completed circuit monitoring and the required brake-function checks. A code that has merely been erased is not confirmation of restored signal integrity or brake assistance.

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.

GM diagnostic parameters, PDF page 53.