P0B78Hybrid/EV Battery Voltage Sense M Circuit Range/Performance
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- MIL on, often without noticeable driving change
- Reduced electric assist or regeneration
- Likely causes
- LikelyMeasuring channel M stuck at a fixed value
- PossibleBattery controller software needing an update
- RareBlock 13 voltage drifting from the pack on two-mode NiMH packs
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €80 – €5,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.