P0B77Hybrid/EV Battery Voltage Sense M Circuit
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P0B77 flags an open or high-resistance tap on voltage sense M, the thirteenth cell channel. GM's 2019 battery energy control module checks seven cases: adjacent cells differing by 1.3 V or more, a balancing gate reading below 36 mV, or splice voltages above 0.1 V (0.5 V at a busbar splice). M is the last channel there with the short 0.6 s window, Type A. The BAS Plus mild hybrid instead needs lines N and N-1 both below 1.4 V plus a jump over 0.5 V within 200 ms, 7 of 10 samples, Type B. Ford's TSB 23-2039 names P0B77 for 2019-2020 Fusion plug-in hybrids that no longer start and prescribes a new traction battery. Range, low and high faults on M carry their own codes.
- What you may notice
- MIL with hybrid or EV system warning
- Vehicle may not start or go into ready mode
- Charging blocked or limited
- Likely causes
- LikelyOpen or corroded tap of sense lead M at the cell connector
- PossibleFault inside the traction battery (Ford Fusion PHEV TSB 23-2039)
- PossibleHigh resistance at a splice or busbar joint in the sense path
- RareDefective cell monitoring board input
- 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: €150 – €7,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.