Medium Speed CAN Communication Bus (-) High

Possible causes

• CommonDamaged, chafed or crushed bus wiring
• CommonCorroded or water-ingressed connector
• CommonOne failed module dragging the whole bus down
• CommonLow battery voltage, which makes modules drop off the bus
• CommonAftermarket electrical work spliced into the bus - alarms, trackers, tow bar wiring, stereos, diagnostic dongles left plugged in
• PossibleCorroded earth point shared by modules on the bus
• PossibleDamaged terminating resistor or a module that contains one

What you might notice

• A long list of lost-communication codes at once
• Several warning lights on together
• Scan tool cannot reach whole groups of modules, or cannot connect at all
• Limp mode, or the vehicle refusing to start
• Instruments, lighting or central locking behaving strangely
• Battery draining overnight, if a module cannot go to sleep

What to check, in order

1. Look for a short to a supply wire where the loom crosses a heavier circuit.
2. Check any aftermarket wiring spliced into the loom.
3. Check battery voltage and state of charge first. A weak battery drops modules off the bus and produces a scatter of communication codes with no wiring fault at all - the single most common false lead on any U-code.
4. Ask what was done to the car recently, and what is plugged into the diagnostic socket. Aftermarket alarms, trackers, tow bar wiring and dongles left permanently connected are a frequent cause of bus faults that appear weeks after the work.
5. Measure the resistance across the two bus lines at the diagnostic socket with the ignition off. On a twin-wire CAN bus with a terminating resistor at each end this is normally around 60 ohms; roughly 120 suggests a break or a missing terminator, and a much lower reading suggests the lines are shorted.
6. Inspect connectors for water and corrosion, particularly any mounted low in the vehicle.
7. Disconnect modules one at a time and re-check. This is slow but it is how a bus fault caused by one failed module is actually found.