P0B77

What it means

P0B77 is the generic code for a fault in the Hybrid/EV battery voltage sense 'M' circuit. The battery control unit compares the voltage reported by one of its voltage sensing circuits with what it expects, or with other sensing circuits. It has seen a signal that is out of range, erratic, missing or implausible. Which part of the pack 'M' refers to depends on the vehicle, so check model-specific information.

Symptoms

• Hybrid or EV system warning light, or malfunction indicator lamp, on
• Reduced power or limp-home mode
• Poor or reduced electric assist, or reduced EV range
• Inaccurate battery state of charge reading
• Car refuses to enter ready mode or will not start
• Engine running more often than normal on a hybrid

Possible causes

• Loose, corroded or damaged connector on the battery voltage sensing harnesslikely
• Damaged, chafed or open/short-circuit wiring in the voltage sense circuitlikely
• Faulty battery monitoring or cell voltage sensing modulepossible
• Imbalanced or degraded battery module or cell giving implausible voltage readingspossible
• Low-voltage supply or earth problem affecting the battery control unit, such as a weak 12 V batterypossible
• Fault inside the battery control unit itself, or outdated software where the manufacturer has a known fixrare
• Water ingress or coolant/condensation contamination inside the battery packrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting any other hybrid battery, insulation or communication codes.
2. Check the 12 V battery state of charge and terminal condition, and look for any low-voltage supply or earth codes.
3. Look up the model-specific description of the 'M' sense circuit and the correct high-voltage isolation procedure; only technicians trained in high-voltage systems should go further.
4. Compare the live pack voltage with individual module or cell voltages on a scan tool, looking for one reading that is off or not changing.
5. Visually inspect accessible battery sensing harnesses and connectors for damage, corrosion, moisture, pin tension and chafing.
6. With the high-voltage system isolated, test the sense circuit for continuity, short to earth, short to supply and resistance against the manufacturer's figures.
7. Check the battery pack for water ingress, physical damage, and cooling or ventilation blockages that could affect the sensors.
8. Check for manufacturer service bulletins or software updates relating to this code for the specific model.

How to fix

• Clean, repair or replace faulty connectors and terminals in the voltage sense circuit.
• Repair or replace damaged wiring in the sense circuit to the manufacturer's procedure.
• Replace the faulty battery monitoring or voltage sensing module if tests confirm it is at fault.
• Rebalance, repair or replace the affected battery module or cell if it is degraded.
• Replace or recharge a weak 12 V battery and repair any supply or earth faults.
• Apply an updated manufacturer calibration to the battery control unit only if a known software fault applies to that model.

Can I keep driving?

Nearby codes

• P0B76GenericHybrid/EV Battery Voltage Sense L Circuit Intermittent/Erratic
• P0B78GenericHybrid/EV Battery Voltage Sense M Circuit Range/Performance