16-02-2025
5 min.
745

U0018 Medium Speed CAN Communication Bus (-) to CAN Communication Bus (+) Short Circuit

U0018 Medium Speed CAN Communication Bus (-) to CAN Communication Bus (+) Short Circuit is a diagnostic trouble code that indicates the vehicle's Medium Speed CAN (Controller Area Network) communication lines have a short circuit between CAN Low (-) and CAN High (+).

The CAN network is the communication system that allows different electronic control modules in the vehicle to exchange information. Modern vehicles contain many control units, including the engine control module, body control module, transmission module, airbag module, ABS module, and infotainment systems.

When the ECM or another control module detects that the CAN Low and CAN High wires are electrically connected together, communication between modules may become unstable or completely interrupted. The vehicle stores the U0018 fault code.

This fault is commonly caused by damaged CAN wiring, shorted connectors, water intrusion, damaged control modules, or electrical interference within the communication network.

What Does U0018 Medium Speed CAN Communication Bus Short Circuit Mean?

The CAN bus uses two communication wires:

CAN High (+)
CAN Low (-)

These two wires transmit digital signals between vehicle modules.

Under normal operation, CAN High and CAN Low maintain specific voltage differences that allow modules to correctly interpret communication messages.

When the two wires become shorted together, the communication signal becomes corrupted.

Possible causes include:

CAN Low wire shorted to CAN High wire
Damaged wiring insulation
Faulty control module
Corroded connectors
Water damage
Incorrect aftermarket electrical installation

What Is the Medium Speed CAN Bus?

Vehicles often use multiple CAN networks operating at different speeds.

The Medium Speed CAN bus is typically used for systems that do not require extremely fast communication.

It may connect modules such as:

Body control modules
Climate control modules
Door modules
Lighting modules
Comfort systems
Interior electronics

The exact modules connected to the Medium Speed CAN network vary depending on the vehicle manufacturer.

How Does the CAN Communication System Work?

The CAN network allows vehicle modules to communicate without requiring separate wiring connections between every component.

For example:

The ABS module can send vehicle speed information.
The engine module can receive operating data.
The body module can control lighting and comfort functions.

Each module sends and receives digital messages through the CAN network.

If communication is interrupted, multiple systems may malfunction because modules cannot exchange required information.

Symptoms of U0018

Symptoms can vary depending on which modules lose communication.

Check Engine Light or Warning Lights

The vehicle may display:

Check Engine Light
ABS warning light
Airbag warning light
Transmission warning light
Stability control warning

Communication Problems Between Modules

Possible issues include:

Multiple fault codes appearing at the same time
Scan tool unable to communicate with certain modules
Missing module information

Electrical System Malfunctions

Drivers may experience:

Incorrect dashboard information
Faulty warning messages
Random electronic failures

Starting Problems

If critical modules cannot communicate, the vehicle may experience:

No-start conditions
Delayed starting
Immobilizer-related problems

Transmission Problems

Communication failures may cause:

Harsh shifting
Transmission warning messages
Limited transmission operation

Intermittent Electrical Issues

Some vehicles may show:

Random system resets
Temporary warning lights
Problems that disappear and return

Common Causes of U0018

Several electrical problems can create a CAN bus short circuit.

Damaged CAN Wiring

The most common cause is damaged communication wiring.

Possible damage includes:

Cut wires
Crushed wiring harnesses
Melted insulation
Wires rubbing against metal components

When insulation is damaged, CAN High and CAN Low wires may contact each other.

Faulty Control Module

A defective module can internally short the CAN network.

Possible faulty modules include:

Body control module
ABS module
Transmission control module
Engine control module
Door control module

Corroded Connectors

Connector problems may include:

Moisture intrusion
Rust
Loose terminals
Poor electrical contact

Corrosion can alter CAN signal quality.

Water Damage

Water entering electrical components may cause:

Internal shorts
Connector corrosion
Communication failures

Common locations include:

Fuse boxes
Control module housings
Door connectors
Interior wiring areas

Aftermarket Electrical Equipment

Incorrectly installed accessories may interfere with CAN communication.

Examples include:

Alarm systems
Remote starters
Multimedia systems
Tracking devices

Battery or Voltage Problems

Low system voltage may cause unstable module communication.

Possible causes:

Weak battery
Charging system problems
Poor electrical connections

How Is U0018 Diagnosed?

CAN communication faults require a systematic electrical diagnosis.

Check for Additional Fault Codes

A diagnostic scan should identify:

Which modules are not communicating
Other CAN-related fault codes
Network communication errors

Multiple communication codes often point toward a network problem.

Check CAN Bus Voltage

Technicians measure:

CAN High voltage
CAN Low voltage
Resistance between CAN lines

Abnormal readings may indicate a short circuit.

Inspect Wiring Harnesses

The CAN wiring should be inspected for:

Physical damage
Pinched wires
Melted sections
Contact with metal components

Inspect Connectors

Connectors should be checked for:

Corrosion
Water damage
Loose pins
Bent terminals

Disconnect Modules to Locate the Fault

Technicians may disconnect control modules one by one to identify which component is causing the CAN short.

A faulty module can restore communication when disconnected.

Check Aftermarket Equipment

Additional electrical devices should be inspected if the problem appeared after installation.

How Is U0018 Fixed?

The repair depends on the source of the CAN communication problem.

Repair Damaged CAN Wiring

Possible repairs include:

Repair broken wires
Replace damaged wiring sections
Restore proper insulation

Repair or Replace Connectors

Repairs may include:

Cleaning corrosion
Repairing terminals
Replacing damaged connectors

Replace Faulty Control Modules

If a module is internally shorted, replacement may be required.

Repair Water Damage

Any source of water intrusion should be corrected to prevent recurring communication faults.

Remove Incorrect Aftermarket Installations

Improperly installed electrical accessories may need repair or removal.

Repair Battery and Charging Problems

Stable voltage supply is necessary for reliable CAN communication.

After repairs:

Clear all fault codes.
Verify CAN communication between modules.
Check system operation.
Perform a road test.
Confirm that U0018 does not return.

Can You Drive With U0018?

Driving with U0018 may be possible in some cases, but it depends on which vehicle systems are affected.

Possible risks include:

Loss of safety system communication
ABS or stability control failure
Transmission problems
Starting issues
Unexpected electrical failures

If multiple warning lights are active or important systems are not working, the vehicle should be inspected immediately.

Is U0018 a Serious Code?

U0018 is generally considered a serious communication system fault.

Unlike a single sensor failure, CAN bus problems can affect multiple vehicle systems at the same time.

A short circuit on the CAN network may prevent important modules from communicating correctly and can lead to safety-related problems.

How to Prevent U0018

Regular electrical maintenance can help prevent CAN communication problems.

Protect Wiring Harnesses

Avoid:

Damaging wiring during repairs
Incorrect routing of electrical cables
Unprotected modifications

Keep Connectors Dry and Clean

Prevent:

Water intrusion
Corrosion
Loose electrical connections

Install Accessories Correctly

Aftermarket electronic devices should be installed by qualified technicians.

Maintain Battery Condition

Stable voltage helps prevent communication problems between modules.

Repair Electrical Problems Early

Small wiring problems can develop into major CAN communication failures.

Final Thoughts

U0018 Medium Speed CAN Communication Bus (-) to CAN Communication Bus (+) Short Circuit indicates that the vehicle has detected a short circuit between the CAN Low and CAN High communication lines.

The most common causes include:

Damaged CAN wiring
Shorted connectors
Faulty control modules
Water damage
Electrical installation problems
Voltage issues

A complete inspection of the CAN network, wiring harnesses, connectors, and connected control modules is required to restore reliable communication between vehicle systems.