Powertrain, generic · Hybrid propulsion

P0B78 Hybrid/EV Battery Voltage Sense M Circuit Range/Performance

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

With P0B78 the controller doubts the reading of channel M rather than its wiring. GM's 2019 hybrid powertrain processor watches for a frozen value: whenever the average of all cells moves by more than 0.022 V, cell M must follow by at least 0.005 V in the same direction, and 35 failures in 40 samples at 100 ms set a Type A fault. The 2012 two-mode trucks map M to nickel-metal hydride block 13 and multiply blocks 13 and 14 by twenty; if that estimate misses the pack voltage by more than 70 V in 160 of 170 samples, the code is stored as Type B. The BAS Plus mild hybrid wants at least half the average movement, 20 of 40 samples, Type B. A plausible but wrong value is often a controller or measuring board issue.

Likely causes

Measuring channel M stuck at a fixed value

Battery controller software needing an update

Block 13 voltage drifting from the pack on two-mode NiMH packs

Symptoms

MIL on, often without noticeable driving change
Reduced electric assist or regeneration

Repair estimate

DifficultyShop_onlyWorkshop job

Parts and labour€80–5,000

Time0.5–6 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

hv battery voltage sense mwiring harnessbms

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com.