U0020 DTC: Medium Speed CAN Bus (-) Open

What Does U0020 Mean?

Diagnostic Trouble Code (DTC) U0020 indicates that the vehicle has detected an open circuit in the Medium-Speed CAN Bus Low (CAN-) communication line. When the CAN Low wire is broken or disconnected, communication between electronic control modules becomes unreliable or stops completely, affecting multiple vehicle systems.

Symptoms

• Check Engine Light illuminated
• Multiple warning lights on the dashboard
• Communication failure with one or more control modules
• Power windows, door locks, or HVAC may stop working
• Instrument cluster warning messages
• ABS or traction control warnings
• Scan tool unable to communicate with some modules

Common Causes

• Broken CAN Low (CAN-) wire
• Loose or disconnected connector
• Corroded terminals
• Damaged wiring harness
• Water intrusion into CAN connectors
• Faulty BCM or other control module
• Improper aftermarket electrical installation

Typical Diagnostic Flow

Battery

│

▼

Scan Tool

│

▼

Inspect CAN Low Wire

CAN High (+) ─────────────── CAN Low (-) ───────────────X

Open Circuit Detected

▼

Repair Wiring or Replace Faulty Module

How to Diagnose U0020

• Scan all modules for additional communication DTCs.
• Inspect the CAN Low wire for damage or breaks.
• Check connectors for corrosion, moisture, or loose terminals.
• Measure CAN bus resistance with the ignition OFF.
• Verify CAN High and CAN Low voltages using a digital multimeter.
• Disconnect suspected modules individually to isolate the fault.

Typical CAN Bus Values

• CAN High: approximately 2.5–3.5 V
• CAN Low: approximately 2.5–1.5 V
• Network resistance: approximately 60 Ω (Ignition OFF)
• Actual specifications vary by vehicle manufacturer.

Estimated Repair Cost

• CAN wiring repair: $80–300
• Connector replacement: $30–150
• Professional CAN diagnosis: $100–250
• Control module replacement: $250–1,000+

Related DTCs

• U0018
• U0019
• U0100
• U0140
• U0155

Frequently Asked Questions

Can I continue driving with U0020?

The vehicle may still run, but communication failures can disable important electronic systems. Repair is recommended as soon as possible.

What usually causes U0020?

The most common causes are an open CAN Low wire, damaged connectors, corrosion, or a failed control module affecting the CAN network.

Will clearing the code fix U0020?

No. The DTC will return until the open circuit is repaired and normal CAN communication is restored.