U0021 DTC: Low Speed CAN Communication Bus (+) Open

What Does U0021 Mean?

Diagnostic Trouble Code (DTC) U0021 indicates that the Powertrain or Body Control Module has detected an open circuit in the Low-Speed CAN Bus High (CAN+) communication line. An open CAN High wire interrupts data transmission between electronic control modules, causing communication failures and multiple electrical system malfunctions.

Symptoms

• Check Engine Light illuminated
• ABS, Airbag, or Traction Control warning lights
• Loss of communication with one or more control modules
• Power windows, mirrors, or door locks may stop working
• HVAC system malfunction
• Instrument cluster warning messages
• Scan tool communication errors

Common Causes

• Open CAN High (CAN+) wire
• Broken wiring harness
• Loose or disconnected connector
• Corroded electrical terminals
• Water intrusion into CAN connectors
• Faulty BCM or another control module
• Improper aftermarket wiring installation

Typical Diagnostic Flow

Battery

│

▼

Scan Tool

│

▼

Inspect Low-Speed CAN High

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

Open Circuit Detected

▼

Repair Wiring or Replace Faulty Module

How to Diagnose U0021

• Read all communication DTCs with a professional scan tool.
• Inspect the CAN High wiring for breaks or damage.
• Check connectors for corrosion, moisture, or loose pins.
• Measure CAN bus resistance with the ignition OFF.
• Verify CAN bus voltage using a digital multimeter.
• Disconnect modules individually if necessary 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
• Values vary depending on vehicle manufacturer.

Estimated Repair Cost

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

Related DTCs

• U0022
• U0023
• U0100
• U0140
• U0155

Frequently Asked Questions

Can I drive with U0021?

Yes, but electronic systems may operate incorrectly or stop communicating. The fault should be repaired promptly.

What usually causes U0021?

The most common causes are an open CAN High wire, damaged connectors, corrosion, or a defective control module connected to the Low-Speed CAN network.

Will clearing the code permanently fix U0021?

No. The DTC will return if the open circuit or communication problem has not been repaired.