P0B99
Hybrid/EV Battery Voltage Sense S Circuit Intermittent/Erratic.
- Generic
- Powertrain
- Generic code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- B
- AreaHybrid propulsion
- 99
- FaultThe specific fault in that area
What it means
P0B99 is the generic code for an intermittent or erratic signal in the voltage sense circuit labelled 'S' of the hybrid or EV traction battery. The battery control unit saw the voltage reading from that circuit jump, drop out or fall outside a believable range compared with the other sense circuits or the pack total. Which cell block or module 'S' refers to depends on the make, model and battery layout, so check the manufacturer data.
Symptoms
- Hybrid system or battery warning light on, possibly with a general warning message
- Reduced power or the car going into a limited performance mode
- Hybrid system not engaging, so the engine runs more or the car will not enter ready mode
- Poor or fluctuating state of charge readings on the display
- Reduced electric range or higher fuel consumption on a hybrid
- Fault that comes and goes with bumps, temperature or vibration
Possible causes
- Loose, corroded or damaged connector on the battery voltage sense harnesslikely
- Damaged, chafed or intermittently open-circuit sense wire between the battery cells or modules and the monitoring unitlikely
- Faulty battery monitoring or cell sensing module giving erratic readingspossible
- Poor busbar or cell terminal connection inside the battery pack causing a fluctuating readingpossible
- Deteriorating battery module with unstable cell voltagepossible
- Moisture ingress or contamination in the battery pack or connectorsrare
- Outdated battery control software where a manufacturer bulletin names it as a causerare
How to diagnose
- Read all stored and pending codes and freeze frame data, noting battery voltage, temperature and whether other battery codes are present
- Check for manufacturer bulletins and the service information for what sense circuit 'S' refers to on this model
- Make sure the high voltage system is isolated and made safe by a suitably trained technician before any work near the battery or orange cabling
- Inspect the sense harness and connectors at the battery pack and monitoring unit for damage, corrosion, moisture, bent pins or poor locking
- Carry out a wiggle test on the harness with live data displayed, watching for the individual block voltages to jump or drop out
- Compare individual cell or block voltages in live data to find the one that is erratic or out of line with the others
- Test the sense circuit for continuity, resistance and shorts to the manufacturer's figures with the system isolated
- Check the monitoring unit's power and ground supplies, and check for battery pack ingress or damage
How to fix
- Repair or replace the damaged sense wiring or connector, and clean or re-seat the terminals
- Repair loose or corroded busbar or terminal connections inside the pack following the manufacturer's procedure
- Replace the faulty battery monitoring or sensing module if the readings are erratic with sound wiring
- Replace or rebalance the affected battery module if its voltage is unstable
- Dry out and treat moisture ingress, and fix the cause of the leak
- Apply a software update only where the manufacturer lists it as the remedy
Can I keep driving?
The car may still drive, but it can drop into reduced power or stop the hybrid system without warning, so book it in soon. If the car loses power, shows a red warning or smells or sounds wrong from the battery, stop safely and get it recovered.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.