P0B78

What it means

P0B78 means the hybrid or EV battery control unit has found a fault in the 'M' battery voltage sense circuit. Its reading is out of range or does not agree with other voltage measurements, such as the neighbouring sensors or the total pack voltage. Which block, module or sensor 'M' refers to depends on the vehicle, so check the manufacturer data for the model.

Symptoms

• Hybrid system or EV warning light on the dashboard
• Reduced power or limp-home mode
• Reduced or no electric assist, or the engine running more often on a hybrid
• Poor or inaccurate state-of-charge reading
• Reduced charging rate or charging that will not start
• Car refusing to enter Ready mode in some cases

Possible causes

• Loose, corroded or damaged connector or terminal on the battery voltage sense wiringlikely
• Damaged, chafed or open-circuit sense wire or harness, including a high-resistance jointlikely
• Imbalanced or weak battery cell or module that produces a voltage reading outside the expected rangepossible
• Faulty battery monitoring or voltage sensing circuit board inside the packpossible
• Low or unstable 12 V supply or poor earth to the battery control unit affecting its measurementspossible
• Fault in the battery control unit itself, or outdated software where the manufacturer has issued a fixrare
• Moisture or coolant ingress in the battery pack affecting the sensing circuitrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting pack voltage, temperature, state of charge and whether the fault is intermittent or constant. Look for related battery, isolation or communication codes.
2. Check the condition and charge of the 12 V battery and the main earths, because low supply voltage can cause false sense readings.
3. Check for manufacturer service bulletins and software updates for the model and code.
4. Look at live data and compare the individual block or module voltages and total pack voltage. Look for one reading that is clearly out of line or fixed.
5. Only a technician trained in high-voltage systems should carry on from here. Isolate the high-voltage system using the manufacturer's procedure and confirm it is safe before touching any high-voltage parts.
6. Inspect the sense harness and connectors for chafing, corrosion, bent or pushed-out pins, water ingress and poor terminal tension, then check continuity and resistance against the wiring diagram.
7. Look for signs of moisture, coolant leaks, damage or overheating in and around the battery pack.
8. Carry out the manufacturer's battery balance or module test to see whether a cell or module is out of range.

How to fix

• Repair or replace damaged wiring and connectors in the sense circuit, and clean or re-seat terminals correctly.
• Replace or repair the faulty voltage sensing circuit board or monitoring module if testing shows it is at fault.
• Rebalance or replace the weak battery module or cells, following the manufacturer's procedure.
• Restore a healthy 12 V supply by charging or replacing the 12 V battery and repairing poor earths.
• Update the battery control unit software only if the manufacturer lists an outdated calibration as the cause, and replace the control unit if it is confirmed faulty.
• Fix any moisture or coolant ingress and replace parts that have been damaged by it.

Can I keep driving?

Nearby codes

• P0B77GenericHybrid/EV Battery Voltage Sense M Circuit
• P0B79GenericHybrid/EV Battery Voltage Sense M Circuit Low