U0003: High Speed CAN Communication Bus (+) Open

What it means

An open 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 uses a positive CAN-High wire and a negative CAN-Low wire to carry messages between modules. U0003 identifies an open on the CAN-High line, which can interrupt communication with modules on the affected network branch.

Common causes

Most likely first.

1. CAN-High wire broken or pin pushed back
2. Corroded module connector
3. Harness damage after repair or accident

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 list which ones fail to communicate, then use the network diagram to identify whether they share the affected CAN branch.
2. Check for related communication codes and note the vehicle conditions when they were set before clearing anything.
3. Inspect the CAN-High wire, connectors, and recent repair or collision areas for a broken conductor, pulled terminal, or pin pushed back in its connector.
4. Check connector fit and continuity of the CAN-High path using the manufacturer’s network diagram and isolation procedure.
5. If the wire and connectors pass, isolate network branches and modules according to service information to locate the open.

Common fixes

• Reseat a CAN connector with a backed-out or loose CAN-High terminal.
• Repair a broken CAN-High wire or damaged harness section.
• Replace a corroded or damaged connector terminal and restore its seal.
• Repair a confirmed open inside a module or gateway connection after the harness checks pass.

Can I keep driving?

An open CAN-High line can cut off affected modules and may cause limp mode or loss of important functions. Do not drive if braking, steering, engine control, or other essential systems are affected; otherwise, keep any trip short and cautious because more functions may drop out.