U0042 DTC: Vehicle Communication Bus A (+) Low
What Does U0042 Mean?
Diagnostic Trouble Code (DTC) U0042 indicates that the Vehicle Communication Bus A CAN High (CAN+) circuit voltage is lower than the expected operating range. Low voltage on the CAN High circuit interferes with data transmission between electronic control modules, causing communication failures and multiple vehicle systems to malfunction.
Symptoms
- Check Engine Light illuminated
- ABS, Airbag, or Stability 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
- Diagnostic scan tool communication failures
- Multiple communication-related DTCs stored
Common Causes
- CAN High wire shorted to ground
- Damaged or pinched wiring harness
- Loose or corroded connectors
- Water intrusion into CAN connectors
- Poor ground connection
- Faulty CAN transceiver inside a control module
- Improper aftermarket electrical modifications
Typical Diagnostic Flow
How to Diagnose U0042
- Read all communication DTCs using a professional scan tool.
- Measure CAN High voltage with a digital multimeter.
- Inspect the wiring harness for damage or contact with ground.
- Check connectors for corrosion, moisture, or loose terminals.
- Inspect vehicle ground connections.
- Disconnect control modules individually if necessary to isolate the faulty module.
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 may 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
- U0040
- U0041
- U0043
- U0100
- U0140
Frequently Asked Questions
Can I drive with U0042?
The vehicle may continue to operate, but communication problems can affect safety systems, body electronics, and convenience features. The fault should be diagnosed and repaired as soon as possible.
What is the most common cause of U0042?
The most common causes are a CAN High wire shorted to ground, damaged wiring, poor ground connections, corroded connectors, or a faulty control module.
Will clearing the DTC permanently fix U0042?
No. Clearing the code only removes it temporarily. The DTC will return until the low-voltage condition on the CAN High circuit has been corrected.