P0B66

What it means

P0B66 means the hybrid or EV battery control unit saw the signal from one of its battery voltage sensing circuits (labelled 'I') read higher than the expected range. This can be a wiring or connector fault, a fault in the sensing hardware, or a real voltage problem in the monitored battery section. Which cells, module or pack section circuit 'I' covers depends on the make, model and battery design.

Symptoms

• Hybrid or EV system warning light, or the malfunction indicator lamp, on
• Reduced power or limp-home mode
• Reduced electric assist, or the engine running more than normal on a hybrid
• Reduced EV range or a state-of-charge reading that jumps about
• Car refusing to enter ready mode or to charge in some cases

Possible causes

• Damaged, corroded or loose wiring or connector in the battery voltage sense circuit, such as a short to voltage or a high-resistance faultlikely
• Faulty battery monitoring unit or cell voltage sensing board inside or attached to the battery packlikely
• Imbalanced or failing battery module or cell group giving a genuinely high voltage readingpossible
• Poor connection at the battery pack busbar or sense lead terminalspossible
• Fault in the battery control unit itself or outdated software where the manufacturer has issued a fixrare
• Moisture or coolant ingress into the battery pack or its harness connectorsrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data. Note any other battery voltage, temperature or insulation codes, as they help to narrow down the circuit involved.
2. Check the manufacturer's information for which cells, module or pack section sense circuit 'I' covers on this model.
3. Visually inspect accessible low-voltage harnesses and connectors at the battery pack for damage, chafing, corrosion, moisture or loose fit. Do not open high-voltage components unless trained and qualified.
4. Using live data, compare individual module or cell group voltages and look for one that is out of line with the rest, or for a reading stuck at a maximum.
5. Have a technician trained in high-voltage systems make the vehicle safe and isolate it following the manufacturer's procedure before any physical work on the pack.
6. Check the sense circuit for continuity, resistance and short to voltage against the wiring diagram, with the system isolated.
7. Check the battery control unit supply and earth connections, and see whether there are any software updates or technical bulletins for this fault.
8. Clear the code after repair and road test, then recheck live data to confirm the fault does not return.

How to fix

• Repair or replace damaged wiring, terminals or connectors in the sense circuit.
• Clean and refit corroded or loose connections, and fix any source of moisture ingress.
• Replace the faulty battery monitoring unit or sensing board as the manufacturer specifies.
• Rebalance or replace the failing battery module or cell group if the voltage is genuinely wrong.
• Update the battery control unit software only if the manufacturer lists it as a fix for this fault.

Can I keep driving?

Nearby codes

• P0B65GenericHybrid/EV Battery Voltage Sense I Circuit Low
• P0B67GenericHybrid/EV Battery Voltage Sense I Circuit Intermittent/Erratic