P0B74

What it means

P0B74 means the hybrid or EV battery control unit saw the signal from one of its battery voltage sense circuits, labelled 'L', reading lower than the expected range. The control unit uses these readings to monitor pack and module voltage and to decide how much power to allow. Which physical circuit 'L' refers to depends on the vehicle and battery design, so check the manufacturer's data for that model.

Symptoms

• Hybrid or EV system warning light on the dashboard
• Reduced power or limp-home mode
• Car will not go to READY or will not start in EV mode
• Hybrid battery charge and discharge limited, or poor fuel economy
• Erratic or inaccurate state-of-charge reading
• Engine running more often than normal on a hybrid

Possible causes

• Loose, corroded or damaged connector on the battery voltage sense circuitlikely
• Open circuit, chafed wiring or high resistance in the voltage sense harnesslikely
• Genuinely low cell or module voltage from a weak or imbalanced battery packpossible
• Fault in the battery monitoring unit or its voltage sensing circuitrypossible
• Poor busbar or sense lead connection at the battery packpossible
• Moisture or coolant ingress into the battery pack or monitoring electronicsrare
• Outdated or faulty control unit software, where the manufacturer has documented it as a causerare

How to diagnose

1. Read all stored codes and freeze frame data. Note battery voltage, state of charge, temperature and whether other battery or voltage-sense codes are present.
2. Check the 12 V battery condition and charging, as low supply voltage can cause false sense faults on some models.
3. Visually inspect the battery pack harness, connectors, and the area around the pack for damage, corrosion, moisture or previous repairs. Only a trained technician following the high-voltage isolation procedure should do this.
4. Using live data, compare individual module or cell voltages and look for one reading that is much lower than the others or that jumps around.
5. Check the manufacturer's wiring diagram to identify which circuit is 'L' on this model, then test continuity, resistance and the reference voltage of that sense circuit.
6. Wiggle-test the harness and connectors with live data on screen to see whether the reading drops out.
7. Check pack balance and capacity using the manufacturer's test procedure, as the method varies by model.
8. Check for applicable software updates or technical bulletins for the vehicle.

How to fix

• Clean, repair or replace damaged connectors and terminals on the voltage sense circuit.
• Repair or replace the faulty section of wiring harness.
• Re-secure or repair loose sense leads or busbar connections at the battery pack.
• Rebalance, repair or replace the affected battery module or pack if cell voltage is genuinely low.
• Replace the battery monitoring unit if testing proves it is faulty.
• Apply a manufacturer-issued software update if one is documented for this fault.

Can I keep driving?

Nearby codes

• P0B73GenericHybrid/EV Battery Voltage Sense L Circuit Range/Performance
• P0B75GenericHybrid/EV Battery Voltage Sense L Circuit High