Damaged, chafed or pinched CAN High and CAN Low wiring shorting together; Water ingress or corrosion in a connector or splice linking the two bus wires; Faulty module internally pulling the bus lines together; Poor previous repair, such as an incorrect splice or aftermarket accessory wired into the bus; Damaged diagnostic socket or its terminals; Failed or faulty termination within a module affecting bus levels.

U0009

High Speed CAN Communication Bus (-) shorted to Bus (+).

Generic
Network
Generic code
High severity

How this code is built

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

What it means

U0009 means the control unit has detected that the high speed CAN bus negative (CAN Low) wire is shorted to the bus positive (CAN High) wire. This fault stops modules exchanging messages properly, so the control unit sets the code when it sees bus voltages or signal levels that match the two lines being linked together.

Symptoms

Multiple warning lights on at once, such as ABS, traction control, airbag or engine management
Instrument cluster gauges dead, flickering or showing dashes
Engine cranks but will not start, or runs in a reduced power mode
Scan tool unable to communicate with one or more modules
Other U-codes for lost communication stored in several modules
Intermittent faults that change when wiring is moved

Possible causes

Damaged, chafed or pinched CAN High and CAN Low wiring shorting togetherlikely
Water ingress or corrosion in a connector or splice linking the two bus wireslikely
Faulty module internally pulling the bus lines togetherpossible
Poor previous repair, such as an incorrect splice or aftermarket accessory wired into the buspossible
Damaged diagnostic socket or its terminalspossible
Failed or faulty termination within a module affecting bus levelsrare

How to diagnose

Read and record all stored codes and freeze frame data from every module that will respond, noting which modules are missing
Check for recent work, accident damage, water ingress or fitted accessories that may have affected the bus wiring
Visually inspect the diagnostic socket, main connectors and wiring along the bus route for chafing, crush damage and corrosion
With the ignition off and battery disconnected, measure resistance between CAN High and CAN Low at the diagnostic socket and compare with the manufacturer's specification, since a short usually reads far below the normal value
Check each wire for continuity and for a short to the other wire, and to ground and supply
Unplug modules one at a time, or split the network at junctions, and recheck resistance to isolate the faulty branch or module; the method depends on the model's network layout
Use an oscilloscope on both bus lines to confirm the signals mirror each other normally after repair
Clear the codes and retest, confirming the fault does not return

How to fix

Repair or replace the damaged section of wiring, using proper soldered and sealed repairs in line with manufacturer guidance
Clean, dry or replace corroded connectors and terminals and cure the source of water ingress
Replace a module confirmed to be internally shorting the bus, and code or program it if the model requires
Remove or correct any faulty aftermarket wiring connected to the bus
Replace the diagnostic socket if its terminals or internal wiring are damaged

Can I keep driving?

High severity

Avoid driving until it is diagnosed, as lost communication can affect safety systems such as ABS, airbags and engine control, and the car may not start or may stall. If it must be moved, do so only a short distance with great care.

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