P0B56

What it means

P0B56 means the hybrid or EV control unit saw the signal from voltage sense circuit 'F' on the high-voltage battery reading lower than expected. 'F' usually refers to one particular cell, module or sensing channel, but which one depends on the make, model and battery design. The fault is in the sensing circuit or its reading, so it may not mean the battery itself is low on charge.

Symptoms

• Hybrid or EV system warning light on the dashboard, often with the malfunction indicator lamp
• Reduced power or limp-home mode
• Poor or erratic state-of-charge reading on the display
• Petrol engine running more than normal on a hybrid, with poorer fuel economy
• Vehicle refusing to go into ready mode or to charge (on a plug-in or EV)
• Battery cooling fan running harder than usual

Possible causes

• Damaged, corroded or loose connector or wiring on the battery voltage sensing harnesslikely
• Faulty battery monitoring unit or sensing board (voltage sense channel reading low or open)likely
• Weak or failed battery cell or module giving a genuinely low voltage reading on that channelpossible
• Poor connection at a battery busbar or sense tap, such as a loose or corroded terminalpossible
• Short to ground in the sense circuit caused by chafed insulation or water ingress in the battery packpossible
• Outdated control unit software where the manufacturer has issued a fix for false voltage sense faultsrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data. Note any other battery voltage, temperature or insulation codes, and whether the fault is current or intermittent.
2. Check for manufacturer service bulletins and software updates for this code on the specific model and battery type.
3. Review live battery data: individual cell or module voltages, pack voltage and the difference between highest and lowest. Look for the 'F' channel reading well below its neighbours or reading zero.
4. Without touching any high-voltage parts, visually inspect the accessible low-voltage connectors and harness at the battery control unit for damage, moisture, corrosion or bent pins.
5. Have a trained technician make the high-voltage system safe using the manufacturer's isolation procedure before any work on the pack. Do not open the battery pack without that training and the correct protective equipment.
6. Once the system is isolated, check the sense wiring for continuity, shorts to ground and connector resistance against the manufacturer's wiring data.
7. Check the busbar and sense tap connections are tight and clean, and check the pack for signs of water or coolant ingress.
8. If the wiring is sound and a cell or module is reading low, run the manufacturer's battery health or balance test to decide between a faulty sensor channel and a weak cell.

How to fix

• Repair or replace damaged sense wiring or connectors, and clean or reseat any corroded terminals.
• Re-tighten or repair loose busbar or sense tap connections to the manufacturer's specification.
• Replace the faulty battery monitoring unit or sensing board if it is giving a false low reading.
• Replace or rebalance the weak battery cell or module, or repair the pack, if testing confirms a genuine low-voltage cell.
• Dry out and seal the pack and fix the source of any moisture ingress.
• Update the control unit software if a manufacturer fix applies to this fault.

Can I keep driving?

Nearby codes

• P0B55GenericHybrid/EV Battery Voltage Sense F Circuit Range/Performance
• P0B57GenericHybrid/EV Battery Voltage Sense F Circuit High