What it means
The BMS detects a circuit fault on voltage sense channel B of the high-voltage battery. Cell-group voltage measurement is unreliable on this channel.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
Voltage sense channel B measures a cell group's voltage in the high-voltage battery so the battery management system can monitor the pack and manage charging and power. P0B40 sets when the system detects a general fault in channel B's measurement circuit, making that cell-group reading unreliable.
Common causes
Most likely first.
- Open or damaged voltage-sense wiring
- Loose or corroded connector
- Fault in the battery voltage-sensing module
Symptoms
- EV/hybrid system warning light on
- Reduced power output or limp mode
- Charging may fail or be slowed
- Reduced electric range
How to diagnose it
- Record freeze-frame data and compare channel B's scan-tool reading with other cell-group voltages and related battery codes.
- Check whether channel B's value is missing, fixed, or implausible, which can help localize the problem to its measurement path.
- Inspect accessible channel B wiring and connectors for damage, looseness, or corrosion.
- Have channel B sense wiring checked for opens and poor connections, and test the voltage-sensing module according to the manufacturer's high-voltage safety procedure.
- If the circuit and module pass, have the relevant cell-group condition assessed using the manufacturer's diagnostic procedure.
Common fixes
- Secure a loose channel B connector or repair accessible damaged wiring.
- Clean or repair corroded channel B connector terminals.
- Repair an open or poor connection in the channel B sense circuit.
- Replace or repair the battery voltage-sensing module if testing confirms a fault.
- Address a verified cell-group problem using the manufacturer's service procedure.
Can I keep driving?
The battery management system may limit power, charging, or electric range while channel B is unreliable. Avoid demanding or extended trips and arrange diagnosis, especially if a warning or reduced-power mode appears.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.