U0004: High Speed CAN Communication Bus (+) Low

What it means

A low signal fault has been detected on the positive 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 bus sends messages over paired CAN-High and CAN-Low wires. U0004 means the CAN-High line is being detected at an abnormally low signal level, which can happen if it is shorted to ground or a network component is pulling it low.

Common causes

Most likely first.

1. CAN-High shorted to ground
2. Internal transceiver failure on a node
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 for communication codes and identify which ones are offline, then map them to the vehicle’s high-speed CAN branches.
2. Inspect the CAN-High harness and connectors for crushed insulation, chafing to ground, water intrusion, or damage near recent repairs.
3. Check battery and module grounds for loose or corroded connections that could affect bus behavior.
4. Following the manufacturer’s procedure, measure CAN-High signals and check for a short to ground with the network safely isolated.
5. If the wiring is not shorted, isolate branches and connected modules as specified to find a failed transceiver or gateway issue.

Common fixes

• Repair CAN-High wiring that is shorted to ground or crushed against the body.
• Clean, reseat, or replace a damaged CAN connector or terminal.
• Correct a poor module ground that is pulling the network signal low.
• Replace a confirmed faulty module or gateway only after the CAN wiring tests pass.

Can I keep driving?

Affected modules may lose communication, trigger limp mode, or disable vehicle functions, so do not drive if essential controls are impaired or warning lights indicate a safety system fault. If the vehicle remains controllable, restrict driving and have the network fault diagnosed promptly.