U0009: High Speed CAN Communication Bus (-) Shorted to Bus (+)

What it means

A short between the positive and negative lines of the high-speed CAN communication bus has been detected. The affected functions depend on the vehicle network layout.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

The high-speed CAN bus uses CAN-H and CAN-L together to carry messages between control modules. U0009 sets when the two signal wires are electrically connected, so their differential communication is disrupted and modules on that network may stop exchanging messages.

Common causes

Most likely first.

1. CAN-H and CAN-L shorted together
2. Pinched harness behind a module
3. Gateway module routing CAN messages incorrectly

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Scan all modules and record freeze-frame data and communication codes to map which high-speed CAN nodes were lost when the short occurred.
2. Inspect accessible high-speed CAN harness sections and module connectors for pinching, crushed insulation, or bent pins that could bridge CAN-H and CAN-L.
3. With the vehicle powered down as specified by the manufacturer, check for continuity between CAN-H and CAN-L and compare with the manufacturer's specification.
4. Inspect recently disturbed areas behind modules or after repairs for a trapped or misrouted paired CAN harness.
5. Use the manufacturer's branch isolation procedure to locate the harness segment or module connector where CAN-H and CAN-L become connected.
6. Assess gateway routing only after confirming the CAN pair is separated and the connected modules are not creating the short.

Common fixes

• Separate and repair CAN-H and CAN-L wires shorted by damaged insulation.
• Replace a pinched harness section behind a module.
• Repair bent or contaminated connector terminals bridging the CAN pair.
• Replace a module only if testing confirms its transceiver internally shorts CAN-H to CAN-L.
• Correct gateway configuration if the physical CAN pair and modules test properly.

Can I keep driving?

Do not drive if the fault affects steering, braking, or other essential functions; otherwise keep any trip short and cautious because the high-speed network may disable modules or cause limp mode. Continued operation can leave additional systems without communication.