U0008: High Speed CAN Communication Bus (-) High

What it means

A high signal fault has been detected on the negative line of the high-speed CAN communication bus. 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 network carries messages between key control modules, and CAN-L is its negative signal wire. U0008 sets when CAN-L is detected unusually high, consistent with the line being pulled toward battery voltage or otherwise unable to maintain its expected signal.

Common causes

Most likely first.

1. CAN-Low shorted to battery voltage
2. Chafed harness against power wire
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 save freeze-frame data and communication codes to see which high-speed CAN nodes were affected when CAN-L went high.
2. Inspect the high-speed CAN harness, especially CAN-L near power wiring, for chafing, pinching, melted insulation, or a poor repair that could connect it to supply voltage.
3. With the vehicle powered down as specified by the manufacturer, check CAN-L for a short to battery or another supply and compare with the manufacturer's specification.
4. Check module and gateway connectors for bent pins, corrosion, or contamination that could bridge CAN-L to a powered circuit.
5. Use the manufacturer's network isolation procedure to determine whether a connected module or gateway is driving CAN-L high.
6. Verify gateway routing or configuration after confirming the CAN-L wiring and connected transceivers are sound.

Common fixes

• Repair CAN-L insulation damaged against a power wire.
• Correct a connector pin or contamination bridging CAN-L to supply.
• Restore a damaged section of high-speed CAN wiring.
• Replace a module with a confirmed CAN transceiver fault that holds CAN-L high.
• Correct gateway routing or configuration if the physical circuit checks out.

Can I keep driving?

Avoid driving if essential controls or safety systems are affected; a high-speed network fault can disable modules or place the vehicle in limp mode. If the vehicle remains controllable, limit driving and have the fault addressed promptly because more systems may drop offline.