P258C: Low vacuum-pump control circuit is a driver-state finding

The standardized P258C label refers to vacuum pump control circuit A low. In the cited GM application, the component is the brake-booster vacuum pump and the monitor classifies the fault as continuity toward ground. “Low” describes an electrical control condition, not a measured amount of brake-booster vacuum.

What the source actually measures

The threshold is an internal driver voltage decision. The table does not give a wire-probing voltage or identify a failed terminal. A low control-circuit finding can guide circuit diagnosis, but it does not prove that the pump's mechanical pumping element has failed.

The same row distinguishes P258A as open and P258D as continuity toward voltage. Preserving those distinctions matters if several faults appear at different times: an open event and a ground-side event should not be rewritten as a single generic “bad pump” diagnosis.

Verification needs the correct kind of result

GM lists a 0.01-second primary interval with a two-driving-cycle recognition strategy. A separate P258B monitor assesses the pressure change during commanded pump activation. Electrical validity and vacuum response are therefore different pieces of evidence, and neither should be assumed from the absence of the other code.

Record the driver fault, command state and companion pressure-performance information for a qualified technician. The source restricts this calibration to GMX020/023 within its LNF-specific section. It supplies no general brake-assist guarantee, and a cleared code is not proof that assistance is available. Any control-circuit repair must be followed by the vehicle's approved functional verification.

Source and applicability

GM 2009 09OBDG02 Engine Diagnostics, LNF-specific section, with the brake-booster pump monitor expressly restricted to GMX020/023 applications. This does not establish broader vehicle fitment or a complete brake service procedure.

General Motors source, physical page 60