U0018 DTC: Medium Speed CAN Bus (-) Shorted to Bus (+)

What Does U0018 Mean?

Diagnostic Trouble Code (DTC) U0018 indicates that the Medium-Speed Controller Area Network (MS-CAN) has detected a short circuit between the CAN High (+) and CAN Low (-) communication wires. Because the CAN network allows electronic control modules to exchange information, a short between these two wires can interrupt communication and cause multiple vehicle systems to malfunction.

Symptoms

• Check Engine Light or Service Warning Light
• Multiple warning lamps illuminated
• Communication failure with one or more modules
• Power windows, locks, or HVAC may stop working
• Instrument cluster warning messages
• Scan tool unable to communicate with some modules
• Intermittent electrical problems

Common Causes

• CAN High and CAN Low wires shorted together
• Damaged or pinched wiring harness
• Water intrusion inside electrical connectors
• Loose, bent, or corroded terminals
• Faulty Body Control Module (BCM)
• Failed control module internally shorting the CAN network
• Incorrect aftermarket electrical installation

Typical Diagnostic Flow

Battery

│

▼

Scan Tool

│

▼

Inspect CAN Wiring

│

▼

CAN High (+)
──────────────

✖

CAN Low (-)
──────────────

Short Circuit Detected

▼

Repair Wiring or Replace Faulty Module

How to Diagnose U0018

• Read all stored DTCs using a professional scan tool.
• Inspect the CAN harness for crushed, cut, or melted wires.
• Disconnect modules one at a time to isolate the short.
• Inspect connectors for moisture or corrosion.
• Measure CAN bus resistance with the ignition OFF.
• Verify CAN High and CAN Low voltage with the ignition ON.
• Repair wiring or replace the failed module if necessary.

Typical CAN Bus Values

• CAN High: approximately 2.5–3.5 V
• CAN Low: approximately 2.5–1.5 V
• Network resistance (ignition OFF): approximately 60 Ω
• Actual values may vary depending on vehicle manufacturer.

Estimated Repair Cost

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

Related DTCs

• U0017
• U0019
• U0020
• U0100
• U0140

Frequently Asked Questions

Can I continue driving with U0018?

The vehicle may still drive, but communication failures can disable important systems such as ABS, traction control, body electronics, or airbags. The problem should be repaired as soon as possible.

Can a bad control module cause U0018?

Yes. Although damaged wiring is more common, an internally failed control module can short the CAN network and trigger U0018.

Will clearing the code fix the problem?

No. If the short circuit remains, the code will return immediately after the network performs another self-test.