P0B79

What it means

P0B79 means the hybrid/EV battery control unit has seen a voltage signal below the expected range on the voltage sense circuit labelled 'M'. This is a circuit-level fault on one of the battery voltage monitoring channels. It does not necessarily mean the battery itself is low or failing. Which cell block, module or sensor 'M' refers to depends on the model and battery design.

Symptoms

• Hybrid or EV system warning light on the dashboard
• Malfunction indicator lamp illuminated
• Reduced power or limp-home mode
• Reduced electric assist or EV range
• Poor or inaccurate battery state-of-charge reading
• In some cases the car will not enter ready mode or will shut down the high-voltage system

Possible causes

• Damaged, loose, corroded or contaminated voltage sense wiring or connector at the battery pack or control unitlikely
• Open circuit or high resistance in the sense lead, fuse link or busbar connection for the affected channellikely
• Fault in the battery monitoring unit or cell sensing circuit for that channelpossible
• Genuine low voltage in the monitored cell block or module caused by an imbalanced or degraded batterypossible
• Water ingress or coolant/condensation contamination inside the battery pack or connectorspossible
• Fault in the main battery control unit, or outdated software where the manufacturer has issued a fixrare

How to diagnose

1. Confirm the code and read freeze frame data, noting the battery state of charge, temperature, pack voltage and whether other battery codes are stored.
2. Check for related codes, such as other voltage sense, cell imbalance or insulation fault codes, and look up the manufacturer's definition of sense circuit 'M' for the specific model.
3. Only a technician trained in high-voltage systems should go beyond this point. Isolate the high-voltage system following the manufacturer's safe procedure before touching any battery connectors or components.
4. Visually inspect the battery pack harness, connectors and mounting for damage, corrosion, moisture, rodent damage or loose pins.
5. Compare live data for individual cell block or module voltages to find the affected channel and see whether the reading is zero, low or out of line with its neighbours.
6. Test the sense circuit for continuity, resistance and short to ground against the wiring diagram, with the system isolated.
7. Check the busbar and sense lead connections for tightness and condition.
8. Check for service bulletins or software updates covering the battery control unit for this model.

How to fix

• Repair or replace damaged sense wiring and clean or replace corroded or damaged connectors.
• Re-secure or replace loose or faulty busbar and sense lead connections.
• Dry out and rectify the source of any moisture ingress, and replace affected components.
• Rebalance, repair or replace the affected battery module if a genuine low voltage is confirmed, as directed by the manufacturer.
• Replace the battery monitoring unit or control unit if the circuit and battery are proven good, and apply any applicable manufacturer software update.

Can I keep driving?

Nearby codes

• P0B78GenericHybrid/EV Battery Voltage Sense M Circuit Range/Performance
• P0B7AGenericHybrid/EV Battery Voltage Sense M Circuit High