P08C5: The paired rod movements behind a synchronizer stuck-on diagnosis

P08C5 is associated here with synchronizer pressure-control valve 1 remaining hydraulically active. Instead of inferring this from coil current, the controller observes the movement caused when the selector redirects an unwanted hydraulic force.

Movement has to form a particular pattern

The documented logic looks within 200 milliseconds of the selector pilot valve's target change. One rod must show at least 1,000 micrometres of unintended movement, while the other shows at least 125 micrometres of relaxation. Which rod supplies each observation changes with the selector's hydraulic target.

That pairing matters. The unexpected movement first makes a valve suspicious; the accompanying relaxation supports the conclusion that the same pressure source was acting on the other route. A single noisy position value is not the whole specified test.

Conditions that distinguish a valid check

The table requires zero force targets for the two rods and excludes electrical faults affecting their position sensors or the relevant synchronizer pressure valves. Reset, error and bypass states also affect eligibility. Without those conditions, apparent movement may not be interpretable using this monitor.

For a meaningful investigation, preserve the selector target, both rod position traces, force commands and electrical fault status together. These observations separate a commanded motion from the unwanted hydraulic effect that the monitor is trying to detect. The source describes the diagnostic principle, not a safe manual procedure for energizing transmission valves or a universal valve replacement instruction.

Source and scope

2025 GM 25OBDG07A Part 2 monitor tables only; model, engine and transmission applicability are not established by the unresolved application-matrix filename join. These are documented monitor criteria, not a universal repair procedure. GM PDF, physical page 1020.