P0B77: sense-M fault needs the exact pack channel map

P0B77 is the voltage-sense circuit M entry in the cited GM battery-controller table. It belongs to the open-sensing monitor family. The code's letter identifies the measurement channel, but does not establish a universal cell number or pack position.

Do not count alphabet letters to locate a cell

The summary lists many channel names across several pages and does not supply a complete physical mapping. Inferring a connector pin or cell location from the letter sequence would add information the source does not provide.

The actual diagnostic strategy compares neighboring readings and evaluates busbar or sensing-power patterns. Those relationships can indicate a measurement fault even when the display appears to implicate a battery cell.

Keep the shared restrictions with the M result

The monitor requires valid communication and excludes specified sensing-module communication, balancing and internal-performance faults. A separate protection self-test must not be running. These conditions determine whether the absence of a repeat code has diagnostic value.

The source leaves its busbar-positive comparison incomplete. This article therefore supplies neither a guessed threshold nor a standalone physical test for that branch.

A qualified technician should identify M from the correct pack documentation, compare its reading with the related channels, and establish the actual sensing-path failure. High-voltage isolation procedures remain necessary for physical work. A stored P0B77 alone cannot distinguish wiring, sensing electronics and genuine cell condition well enough to authorize replacement.

Application limits

Document-level 2015 GM diagnostic table; per-code vehicle/engine/calibration applicability not established.

Source

15OBDG01 Hybrid Diagnostics, physical PDF pages 635, 636.