U0021
Low Speed CAN Communication Bus (+) Open
- Severity
- Low
- Driving
- Drivable — book it in
The positive line of the low-speed CAN bus is open - broken, disconnected, or interrupted somewhere along its length. The two bus wires carry mirror-image signals, and modules read the difference between them to reject electrical noise; with one wire open that comparison cannot be made. It usually carries body, comfort and infotainment systems rather than anything that changes how the car drives.
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
- Check continuity along the positive bus wire between two modules that should both be reachable.
- Look at what was fitted or disturbed most recently. A broken bus wire on a car nobody has touched is far less likely than one at a connector someone opened.
- 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.
- 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.
- 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.
- Inspect connectors for water and corrosion, particularly any mounted low in the vehicle.
- 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.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
U00xxU01xxU02xxU03xxU04xxU05xxU06xxU07xxU08xxU09xxU0AxxU0BxxU0CxxU0DxxU0ExxU0FxxU10xxU11xxU12xxU13xxU14xxU15xxU16xxU17xxU18xxU19xxU1AxxU1BxxU1CxxU1DxxU1ExxU1FxxU20xxU21xxU22xxU23xxU24xxU25xxU26xxU27xxU28xxU29xxU2AxxU2BxxU2CxxU2DxxU2ExxU2FxxU30xxU31xxU32xxU33xxU34xxU35xxU36xxU37xxU38xxU39xxU3AxxU3BxxU3CxxU3DxxU3ExxU3Fxx
Network electrical
U0001U0002U0003U0004U0005U0006U0007U0008U0009U0010U0011U0012U0013U0014U0015U0016U0017U0018U0019U0020U0021U0022U0023U0024U0025U0026U0027U0028U0029U0030U0031U0032U0033U0034U0035U0036U0037U0038U0039U0040U0041U0042U0043U0044U0045U0046U0047U0048U0049U0050U0051U0052U0053U0054U0055U0056U0057U0058U0059U0060U0061U0062U0063U0064U0065U0066U0067U0068U0069U0070U0071U0072U0073U0074U0075U0076U0077U0078U0079U0080U0081U0082U0083U0084U0085U0086U0087U0088U0089U0090U0091U0092U0093U0094U0095U0096U0097U0098