P0B91

What it means

P0B91 means the hybrid or EV control unit found the voltage reading from battery voltage sense circuit 'R' implausible or outside its expected range. The reading may disagree with other cell, module or pack voltage readings, or it may be stuck, erratic or out of limits. Which cell block or sensing point circuit 'R' refers to depends on the model and battery design, so check the manufacturer's service information.

Symptoms

• Hybrid or EV system warning light or malfunction indicator lamp on
• Reduced power or limited acceleration (limp mode)
• Reduced electric assist or lower EV range
• Hybrid battery charge level behaving erratically or jumping
• Car slow to start, or in some cases refusing to go to READY
• Other hybrid battery voltage or cell balance codes stored alongside

Possible causes

• Damaged, corroded or loose wiring or connector at the battery voltage sense circuit (open circuit, high resistance or short)likely
• Faulty voltage sensing channel or fault in the battery monitoring unit or battery ECUpossible
• Weak or imbalanced battery cell or module causing a genuinely out-of-range voltagepossible
• Poor contact at the high-voltage battery busbar or sense lead terminals, including heat or moisture damagepossible
• Low-voltage supply or earth problem affecting the battery monitoring electronicspossible
• Outdated control unit software where the manufacturer has a known fix for this faultrare

How to diagnose

1. Read all stored and pending codes and note freeze frame data, such as pack voltage, temperature, state of charge and drive state, to see when the fault set.
2. Check for related codes (other voltage sense circuits, cell balance, battery temperature, insulation) and check the manufacturer's service information for what sense circuit R covers on this model.
3. Check for technical service bulletins and available software updates for the battery or hybrid control unit.
4. Check the 12 V battery condition and the earths and supplies to the battery monitoring unit, as low-voltage faults can cause false readings.
5. With the high-voltage system made safe by a trained technician following the manufacturer's procedure, visually inspect the sense wiring and connectors for damage, corrosion, moisture, heat marks and poor terminal tension.
6. Use a scan tool to compare individual cell or module voltages with the total pack voltage and look for the reading on circuit R that differs from the others.
7. Check the wiring continuity and resistance of the sense circuit against the specification, wiggling the harness while monitoring live data for dropouts.
8. Carry out the manufacturer's battery health or cell balance test to separate a sensing fault from a genuinely weak module.

How to fix

• Repair or replace damaged wiring, terminals or connectors on the sense circuit, making sure of proper contact and sealing.
• Clean or re-secure loose or corroded busbar and sense lead connections following the manufacturer's procedure.
• Replace the faulty battery monitoring unit or battery control unit if the sensing channel has failed, and complete any required setup.
• Recondition, rebalance or replace the affected battery module or cell block if its voltage is genuinely out of range.
• Correct any 12 V supply or earth faults found.
• Install the latest manufacturer control unit software if a known fault is addressed by an update.

Can I keep driving?

Nearby codes

• P0B90GenericHybrid/EV Battery Voltage Sense R Circuit
• P0B92GenericHybrid/EV Battery Voltage Sense R Circuit Low