P0B8C: block 17 plausibility requires a block 18 comparison

P0B8C is assigned to block 17 voltage rationality in the GM hybrid parameter document. Its generic voltage-sense Q range/performance wording leaves out the relationships that make the diagnosis useful. The actual test compares block measurements with a separate pack-voltage report.

Two comparisons, one qualified condition

The controller checks the absolute difference between twenty times block 17 voltage and pack voltage. A difference greater than 70 V must occur together with the same excessive difference for block 18. That conjunction is part of the row, not an optional comparison added by a repair guide.

The monitor requires 160 failures within 170 samples at 100 ms intervals. A brief mismatch is therefore different from the sustained sample pattern used to mature this code.

Recognize when rationality is unavailable

GM requires valid block 17 input, RUN mode, the specified low-voltage supply window and more than 200 ms after contactor closure. Several block-circuit and pack-voltage faults inhibit the calculation. Their status helps explain why a later scan may not contain a new performance result.

Preserve block 17, block 18 and total-pack readings together. The source does not decide whether the underlying problem is in a sensing path or actual battery condition. Diagnosis and verification require the correct hybrid service procedure, and the calculation must never be used as an improvised instruction for high-voltage probing.

Source and applicability

GM 2010 10OBDG10 Hybrid Diagnostics, cited BPCM or hybrid-controller monitor only. Historical calibration values are not universal specifications; no per-code model, engine or hardware fitment has been established. GM PDF, physical page 231.