Damaged, chafed, corroded or loose wiring or connectors on the bus network; Low battery voltage or poor power and earth supply to a module on the bus; A faulty module on the network interfering with or pulling down the bus signal; Water ingress or corrosion in a module connector or junction point; Poorly fitted aftermarket electrical equipment such as alarms, trackers or audio units tapped into the bus wiring; Open or incorrect termination resistance on the bus; Outdated or faulty module software, where the manufacturer lists it as a known cause for the model.

U0029

Vehicle Communication Bus A Performance.

Generic
Network
Generic code
Medium severity

How this code is built

U: SystemNetwork
0: Defined bySAE, the same on every car
0: AreaNetwork area 0
29: FaultThe specific fault in that area

What it means

U0029 means the control unit has detected that Vehicle Communication Bus A is not performing correctly. Messages on the network may be corrupted, distorted, intermittent or at the wrong signal levels, even though the bus has not necessarily failed completely. Which bus is 'A' and which modules sit on it depends on the manufacturer and model.

Symptoms

Multiple warning lights on at once, such as ABS, airbag, engine or traction control
Intermittent loss of gauges, radio, or other electrical functions
Scan tool struggles to communicate with one or more modules
Engine misfiring to start, or running in limp mode
Other communication (U-code) faults stored in several modules
Battery drain or modules not going to sleep

Possible causes

Damaged, chafed, corroded or loose wiring or connectors on the bus networklikely
Low battery voltage or poor power and earth supply to a module on the buslikely
A faulty module on the network interfering with or pulling down the bus signalpossible
Water ingress or corrosion in a module connector or junction pointpossible
Poorly fitted aftermarket electrical equipment such as alarms, trackers or audio units tapped into the bus wiringpossible
Open or incorrect termination resistance on the busrare
Outdated or faulty module software, where the manufacturer lists it as a known cause for the modelrare

How to diagnose

Read all stored codes from every module and note freeze frame data, looking for a pattern of which modules are affected or losing communication
Check battery condition and charging voltage, as low or unstable voltage commonly upsets network communication
Visually inspect wiring, connectors and junction points on the bus for damage, chafing, water or corrosion, including areas where previous repairs or accessories were fitted
Check the fuses and the power and earth supplies to the affected modules
Look for any non-standard electrical accessories and disconnect them to see if the fault clears
Using the manufacturer wiring diagram, identify which bus is 'A' on this model and measure its resistance and signal with the battery disconnected or with an oscilloscope as appropriate
Unplug modules one at a time, following the manufacturer procedure, to see if the bus signal returns to normal and isolate a faulty unit
Check for manufacturer software updates or technical bulletins relating to network faults on this model

How to fix

Repair or replace damaged wiring and corroded or loose connectors, and seal them against moisture
Recharge or replace a weak battery and repair any charging system or earth fault
Replace or reprogram a faulty module once it has been confirmed as the cause
Remove or correctly rewire any badly fitted aftermarket equipment
Update module software if an outdated version is a known cause for this vehicle
Clear the codes after repair and confirm they do not return on a test drive

Can I keep driving?

Medium severity

The car may be driveable, but communication faults can affect safety systems such as ABS, airbags and stability control, and can lead to a no-start. Have it diagnosed soon and avoid long journeys if warning lights are showing or the car runs poorly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.