P258A: Open brake-vacuum pump control circuit in a limited GM application

P258A identifies an open vacuum-pump control circuit in the cited GM implementation. The component is the brake-booster vacuum pump, and the table expressly limits this monitor to GMX020/023 applications within its LNF-specific section. That restriction should not be expanded into a universal vehicle or model claim.

An electrical check is not a vacuum measurement

The controller uses an internal output-driver voltage assessment for circuit continuity. GM does not publish an external voltage limit in this row, so there is no supported number here for probing a connector or condemning the pump motor.

The companion P258C and P258D entries distinguish continuity toward ground and toward voltage. P258A is the open-circuit branch. Recording the exact code matters because these categories do not point to identical circuit behavior.

Separate continuity from pump performance

P258B is a different monitor on the same page: it evaluates pressure change during pump activation. Passing an electrical check would not, by itself, prove that the pump produced the expected vacuum response. Conversely, an invalid control circuit can make a functional pressure conclusion premature.

The electrical table lists a 0.01-second primary interval and a two-driving-cycle strategy with continuous or accumulated failure timing. Verification should establish a completed continuity check under the vehicle's required conditions, followed by any applicable functional assessment.

Because this pump supports brake assistance, code clearing or a quiet pump is not evidence that braking performance is acceptable. Circuit diagnosis and functional confirmation require the relevant professional procedure; the monitor summary does not authorize bypassing the pump control.

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