Powertrain, generic · Hybrid propulsion

P0B77 Hybrid/EV Battery Voltage Sense M Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Open or corroded tap of sense lead M at the cell connector
Fault inside the traction battery (Ford Fusion PHEV TSB 23-2039)
High resistance at a splice or busbar joint in the sense path
Defective cell monitoring board input

Symptoms

  • MIL with hybrid or EV system warning
  • Vehicle may not start or go into ready mode
  • Charging blocked or limited

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€150–7,000
Time1–10 h

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

Parts involved

hv battery voltage sense mwiring harnessbms

Related codes

P0B78P0B79P0B7AP0B7BP0B72P0B7C

References: gsi.ext.gm.com, gsi.ext.gm.com, static.nhtsa.gov.