Diagnostic Aids

Suspected intermittents or opens at connectors can be detected with diagnostic service probe which permits voltage to be measured on wires without unplugging connectors. Ensure all terminals make good connection.
When diagnosing intermittent problems, select MALF HISTORY from Tech 1 scan tester ABS menu. Supplemental data can be used to duplicate problem.
Brake switch is normally held open by brake pedal. Pressing brake allows switch to close.

Fig 1: Code 91 - Schematic

Courtesy of GENERAL MOTORS CORP.

Fig 2: Code 91 - Diagnostic Flowchart

Courtesy of GENERAL MOTORS CORP.