P0C77: A precharge rise can be too fast for the expected sequence

P0C77 reports a precharge process that rises too quickly in this GM monitor. The documented condition is bus voltage exceeding 95 percent of battery voltage in less than 50 milliseconds from the start of precharge. The code therefore concerns a sequence, not simply a high voltage measured after charging is complete.

The starting condition matters

The bus must have been below 40 volts before precharge began, and the current and bus-voltage sensors must be valid. Those prerequisites prevent the timing result from being interpreted without knowing where the sequence started and whether its measurements were available.

The controller evaluates this check once per precharge event. A later steady bus voltage does not reproduce the original rise-time information. Preserve the event status and voltage history where the vehicle's diagnostics provide them.

Do not confuse fast and slow precharge

The neighboring P0C78 monitor examines failure to reach the required voltage promptly or excessive current during the sequence. A P0C77 event must not be diagnosed using the slow-precharge logic merely because both descriptions mention contactors.

This table does not identify a stuck component or prescribe a replacement by itself. A qualified high-voltage technician must establish the actual circuit, contactor state and measurement validity using the correct vehicle procedure. Do not attempt manual precharge or bypass an interlock to reproduce the monitor. The historical timing limits are diagnostic context, not safe-work instructions.

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.