High Speed CAN Communication Bus Performance

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. Expect this to be intermittent. Note whether it correlates with bumps, weather or engine temperature, because that narrows the search more than any measurement.
2. Suspect a marginal connection before a failed module. A wire that is nearly broken, or a pin that is nearly seated, behaves exactly like this.
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.