P0B51

What it means

P0B51 is the generic code for a low signal in the hybrid or EV traction battery voltage sense 'E' circuit. The battery control unit saw the voltage reading from that sense circuit fall below the expected range, which points to a circuit or sensor fault rather than proof that the battery itself is flat. Which voltage sense circuit 'E' refers to (a cell group, module or pack-level measurement) depends on the make, model and battery design, so check the manufacturer's information.

Symptoms

• Hybrid or EV system warning light, or malfunction indicator lamp, on
• Reduced power or limp-home mode
• Car refuses to go to READY or will not start in EV mode
• Poor or erratic state-of-charge reading on the display
• Engine running more often than normal on a hybrid
• Charging limited or not starting on a plug-in model

Possible causes

• Damaged, corroded or loose connector or terminal on the battery voltage sense circuitlikely
• Open circuit or short to ground in the sense wiring or harness between the battery modules and the battery control unitlikely
• Faulty voltage sensing input on the battery monitoring unit or module monitoring boardpossible
• A genuinely low or weak cell group or module reading low voltagepossible
• Poor connection at the high-voltage battery busbars or loose module connections affecting the measurementpossible
• Outdated or faulty battery control unit software where the manufacturer has a known issuerare

How to diagnose

1. Read all stored and pending codes and the freeze frame data; note state of charge, pack voltage, temperature and whether other battery codes are present
2. Check for manufacturer bulletins and software updates for this code on the specific model
3. Follow the manufacturer's high-voltage safety procedure and make sure only a suitably trained technician carries out any work on the battery or high-voltage system
4. Inspect the sense wiring, connectors and terminals at the battery control unit and battery pack for damage, corrosion, moisture, chafing or loose pins
5. Use live data to compare the individual cell or module voltages and look for one that is low, flat or reading differently from the rest
6. Check the sense circuit for continuity, short to ground and resistance against the wiring diagram, with the system isolated
7. Check the 12 V supply and earths to the battery control unit, as low supply voltage can cause false readings
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, connectors or terminals in the voltage sense circuit
• Clean and secure corroded or loose connections, and correct any moisture ingress
• Replace the faulty battery monitoring unit or module sensing board if testing confirms it has failed
• Rebalance, repair or replace the weak battery module or cell group if the low reading is real, as per the manufacturer's procedure
• Update the battery control unit software if an outdated calibration is a known cause for the model
• Restore 12 V supply or earth faults that affect the battery control unit

Can I keep driving?

Nearby codes

• P0B50GenericHybrid/EV Battery Voltage Sense E Circuit Range/Performance
• P0B52GenericHybrid/EV Battery Voltage Sense E Circuit High