P0C78: Slow precharge has voltage and sustained-current branches

P0C78 is the slow-precharge diagnostic in this GM hybrid controller. The source describes two branches: bus voltage failing to reach 95 percent of battery voltage within 700 milliseconds, or battery current remaining above a defined comparison for too long while that voltage target is still being approached.

Preserve both voltage and current evidence

The current branch compares battery current with battery voltage divided by 27.63 and requires the excess to persist beyond 87.5 milliseconds. This is a controller-specific relationship, not a general charging-current limit for every battery. The current sensor must be valid for that branch.

The voltage branch requires valid battery- and bus-voltage measurements. The check is executed once per precharge event, so a steady reading after the event cannot substitute for its rise-time and current history.

Separate the failed branch from a parts assumption

A workshop should establish which branch explains the record and retain the relevant sensor-validity states. The code does not by itself distinguish every contactor, load, wiring or measurement issue in the high-voltage circuit. The fast-precharge diagnostic uses different timing logic and should not be substituted.

No manual precharge, interlock bypass or live probing procedure is supplied here. A qualified high-voltage technician must use the exact vehicle service information to evaluate the sequence safely. The goal of verification is a valid, completed precharge assessment with credible measurements, not only cleared fault memory.

Source and applicability

15OBDG01 Hybrid Diagnostics, cited controller and monitor only. Historical calibration limits are not universal specifications; per-code vehicle, engine and hardware applicability must be verified.

GM source, physical page 554 · GM source, physical page 555.