What it means
The BMS detects a circuit low fault on voltage sense channel M 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
The BMS reads a cell group’s voltage through battery voltage sense channel M. P0B79 sets when the channel M circuit falls below its expected range, leaving the BMS without a dependable measurement for that group.
Common causes
Most likely first.
- Sense wire shorted to ground
- Open in sense wire
- Connector pins corroded - high contact resistance
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 BMS live readings for channel M with adjacent channels to confirm whether M is unusually low or absent.
- With the hybrid or EV system safely shut down per the manufacturer’s procedure, inspect channel M’s sense wiring and connectors for damage, corrosion, moisture, or a loose fit.
- Check the channel M connector pins for corrosion or poor contact that could pull the signal down.
- Have a qualified hybrid technician test channel M wiring for a short to ground or an open, using the manufacturer’s circuit-test and isolation procedures.
- If the wiring and connector pass, compare the channel M circuit at the battery monitoring module to determine whether its input is faulty.
Common fixes
- Clean and properly secure a corroded or loose channel M connector.
- Repair an open or damaged section of channel M sense wiring.
- Repair a channel M wire shorted to ground and confirm the BMS reading returns to a plausible value.
- Repair or replace the battery monitoring module if circuit tests confirm an internal channel M fault.
Can I keep driving?
Avoid relying on electric assist or making long trips if power is reduced or charging is limited; the vehicle may restrict operation because the channel M cell-group voltage is not trustworthy. Arrange prompt diagnosis, and stop safely if a battery warning escalates or the vehicle behaves unpredictably.
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.