U0011
Medium Speed CAN Communication Bus Performance
- Severity
- Low
- Driving
- Drivable — book it in
The medium-speed CAN bus is working, but not well - messages are being lost, delayed or corrupted, and modules are having to repeat themselves. Degraded performance usually means a marginal connection or a partially damaged wire rather than an outright break, which is why the symptoms tend to come and go. It usually carries body and convenience 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
- Expect this to be intermittent. Note whether it correlates with bumps, weather or engine temperature, because that narrows the search more than any measurement.
- 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.
- 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