U0028 DTC: Low Speed CAN Communication Bus (-) High

What Does U0028 Mean?

Diagnostic Trouble Code (DTC) U0028 indicates that the Low-Speed CAN Bus Low (CAN-) circuit voltage is higher than the expected operating range. Excessive voltage on the CAN Low circuit disrupts communication between electronic control modules and can result in multiple network-related faults.

Symptoms

• Check Engine Light illuminated
• ABS, Airbag, or Stability Control warning lights
• Communication failure with one or more modules
• Power windows, mirrors, or door locks may malfunction
• HVAC system may not operate correctly
• Instrument cluster warning messages
• Scan tool communication errors

Common Causes

• CAN Low wire shorted to battery voltage
• Damaged or pinched wiring harness
• Loose, bent, or corroded connector terminals
• Water intrusion inside CAN connectors
• Faulty CAN transceiver inside a control module
• Charging system overvoltage
• Improper aftermarket electrical modifications

Typical Diagnostic Flow

Battery

│

▼

Professional Scan Tool

│

▼

Measure CAN Low Voltage

Normal CAN Low (-) ────────────── U0028: ────────────── ▲ High Voltage

CAN- Voltage Too High

▼

Repair Wiring or Replace Faulty Module

How to Diagnose U0028

• Read all communication DTCs using a professional scan tool.
• Measure CAN Low voltage with a digital multimeter.
• Inspect the wiring harness for contact with battery voltage.
• Check connectors for corrosion, moisture, or damaged terminals.
• Verify charging system voltage is within specification.
• Disconnect control modules individually if needed 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 values vary depending on the 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

• U0025
• U0026
• U0027
• U0100
• U0140

Frequently Asked Questions

Can I drive with U0028?

The vehicle may continue to operate, but communication problems can affect body, comfort, and safety systems. Diagnosis and repair should not be delayed.

What is the most common cause of U0028?

The most common causes are a CAN Low wire shorted to battery voltage, damaged wiring, charging system faults, corroded connectors, or a defective control module.

Will clearing the DTC permanently fix U0028?

No. The code will return until the excessive voltage condition on the CAN Low circuit has been corrected.