U0003

What it means

U0003 means the control unit has detected an open circuit on the positive (CAN High) wire of the high speed CAN communication bus. The module reporting it can no longer exchange messages reliably with other modules on that network. The fault is usually in the bus wiring, a connector or a module on the network, rather than in the module that stored the code.

Symptoms

• Multiple warning lights on at once, such as ABS, traction control, airbag or engine management
• Instrument cluster gauges not working or behaving erratically
• Engine cranks but will not start, or starts and stalls
• Scan tool cannot communicate with one or more modules
• Gearbox stuck in one gear or in limp mode
• Intermittent faults that come and go with vibration or when the car is moved

Possible causes

• Open circuit, broken or chafed CAN High wire in the harnesslikely
• Corroded, loose or pushed-out pin in a connector on the CAN networklikely
• Faulty or disconnected module that has failed internally and is affecting the buspossible
• Open or missing terminating resistance in one of the end-of-line modules or in the harnesspossible
• Damage from previous repair work, accident or water ingress at a junction or splicepossible
• Poor power or earth supply to a module causing it to drop off the networkrare

How to diagnose

1. Read all stored codes from every module and note which modules report lost communication; read freeze frame data and check whether the fault is current or historic
2. Check battery condition and charging voltage, as low voltage can cause communication codes and mask the real fault
3. Visually inspect the wiring and connectors around the diagnostic socket and any area with recent repair, damage or water ingress
4. Using the manufacturer wiring diagram, identify which modules are on the high speed CAN network and where the bus junctions and end-of-line terminations are
5. With the ignition off and battery disconnected as the procedure allows, measure resistance between CAN High and CAN Low at the diagnostic socket and compare with the specified value, which depends on the model
6. Check continuity of the CAN High wire end to end, flexing the harness gently to reveal intermittent breaks
7. Unplug modules one at a time as the manufacturer procedure allows, to find one that is pulling down or breaking the bus, and check each module's power and earth supply
8. Clear the codes and retest to confirm that the fault does not return

How to fix

• Repair the broken or chafed CAN High wire using the correct soldered and sealed repair method, keeping the twisted pair arrangement
• Clean, repair or replace corroded or damaged connectors and terminals
• Restore power and earth supplies to any module with a poor connection
• Replace a faulty module that is confirmed as the cause, then code or programme it as the manufacturer requires
• Replace the harness section if the damage is extensive or the terminating resistance is faulty

Can I keep driving?

Nearby codes

• U0002GenericHigh Speed CAN Communication Bus Performance
• U0004GenericHigh Speed CAN Communication Bus (+) Low