P0B77 code meaning
Hybrid/EV Battery Voltage Sense M Circuit
Powertrain system Generic code P0Bxx codes Check engine light
What P0B77 means
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.
P0B77 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyWorkshop only
- Typical labor1 to 10 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P0B77
- MIL with hybrid or EV system warning
- Vehicle may not start or go into ready mode
- Charging blocked or limited
Common causes of P0B77
- Most 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
- Less likelyDefective cell monitoring board input
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Hv battery voltage sense mWiring harnessBms
What each part of P0B77 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| B | Area: hybrid propulsion. |
| 77 | The exact fault in that area. |
What to do next
- Note P0B77 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0B77
What does P0B77 mean?
P0B77 means "Hybrid/EV Battery Voltage Sense M Circuit". P0B77 flags an open or high-resistance tap on voltage sense M, the thirteenth cell channel. It is a generic code, so it means the same on every make.
Is it safe to drive with P0B77?
Drive gently and only to a workshop. With P0B77 the car can go into limp mode (reduced power) to protect itself.
Will P0B77 fail an emissions test?
Yes, it can. P0B77 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0B77 to fix?
Difficulty: workshop only. A typical repair takes 1 to 10 hours of work. This is a job for a workshop.
Will P0B77 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.