U0015

What it means

U0015 means the control unit has detected that the negative (low) wire of the medium speed CAN communication bus is open circuit. The module that set the code can no longer see the expected voltage and signal activity on the CAN low line, so messages between modules may be corrupted or lost. Which bus counts as medium speed, and which modules sit on it, depends on the vehicle make and model.

Symptoms

• Multiple warning lights on at once, such as ABS, airbag, traction control or engine management
• Instrument cluster gauges dropping to zero or behaving erratically
• Features on the affected bus not working, such as climate control, infotainment or central locking
• Engine misfires, runs in limp mode or fails to start in some cases
• Diagnostic tester unable to communicate with some modules
• Battery drain caused by modules not going to sleep

Possible causes

• Damaged, chafed or broken CAN low wire in the harness, often near hinges, door shuts or areas with movementlikely
• Corroded, loose or pushed-back pins in a connector on the bus, or water ingress in a connectorlikely
• Open circuit at a junction connector or splice in the CAN networkpossible
• Faulty control unit with an internal CAN transceiver fault pulling the low line openpossible
• Poor power or earth supply to a module on the bus causing it to drop off the networkpossible
• Previous accident or aftermarket wiring repair or accessory fitted to the CAN linesrare

How to diagnose

1. Read all stored codes from every module and note which modules report U0015 or other lost-communication codes, then check freeze frame data and whether the fault is current or intermittent
2. Check the vehicle wiring diagram to identify which modules and connectors are on the medium speed bus for this model
3. Visually inspect the CAN wiring, connectors and any previous repairs for damage, corrosion, water ingress or pushed-out pins
4. Check battery condition and voltage, and the power and earth supplies to the affected modules
5. With the ignition off and battery disconnected as the procedure directs, measure the resistance between the CAN high and low lines at the diagnostic socket and compare it with the manufacturer's specification
6. Test continuity of the CAN low wire from end to end, wiggling the harness to reveal intermittent breaks
7. Use an oscilloscope to view the CAN low waveform and compare it with CAN high to find which section or module is at fault
8. Isolate modules one at a time, following the manufacturer's procedure, to see if a faulty unit is affecting the bus

How to fix

• Repair or replace the damaged section of CAN low wiring using proper soldered and sealed repairs, keeping the twisted pair arrangement
• Clean, repair or replace corroded or damaged connectors and terminals, and fix any water leaks causing ingress
• Replace a faulty junction connector or splice point
• Replace or repair a faulty control unit once the wiring and supplies are proven good, with coding or programming as the manufacturer requires
• Correct power or earth supply faults to the affected modules

Can I keep driving?

Nearby codes

• U0014GenericMedium Speed CAN Communication Bus (+) High
• U0016GenericMedium Speed CAN Communication Bus (-) Low