Chafed or pinched CAN High and CAN Low wires rubbing together inside a harness loom, often near a flex point or pass-through grommet; Water intrusion into a connector or module housing causing corrosion that bridges the two data lines; A failed module on the medium-speed bus internally shorting the CAN lines together; Aftermarket accessories or poor previous repairs splicing into the bus wiring and creating a bridge; Damaged or improperly seated connector pins allowing the terminals to touch inside the housing; Rodent damage to the harness insulation exposing and shorting the twisted pair

DTC codes / U00xx · Network wiring and bus faults / U0018

U0018Medium Speed CAN Communication Bus (shorted to Bus)

Q: What does code U0018 mean?

Medium Speed CAN Communication Bus (shorted to Bus). U0018 is a generic network code indicating that the medium-speed Controller Area Network (CAN) data bus has been detected as shorted to the bus itself, meaning the CAN High and CAN Low lines are electrically bridged together. The computer or gateway module monitors the differential voltage between these two wires and sets this code when the expected signal separation collapses, preventing normal communication between modules on that bus segment.

U0018 is a generic network code indicating that the medium-speed Controller Area Network (CAN) data bus has been detected as shorted to the bus itself, meaning the CAN High and CAN Low lines are electrically bridged together. The computer or gateway module monitors the differential voltage between these two wires and sets this code when the expected signal separation collapses, preventing normal communication between modules on that bus segment.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaNetwork wiring and bus faults

Symptoms you may notice

Multiple warning lights illuminated across the dashboard, often including the anti-lock brake, airbag, and traction control lamps
The engine may still run and drive normally if the fault is isolated to a non-critical medium-speed segment
Intermittent or complete failure of features such as the radio, climate control, or power windows
A no-start condition if the shorted bus prevents the engine control module from receiving necessary data
Often no symptoms besides the warning light when the short is intermittent or confined to a single module
Modules on the affected bus may drop offline and reappear unpredictably while driving

Common causes, most likely first

Chafed or pinched CAN High and CAN Low wires rubbing together inside a harness loom, often near a flex point or pass-through grommet
Water intrusion into a connector or module housing causing corrosion that bridges the two data lines
A failed module on the medium-speed bus internally shorting the CAN lines together
Aftermarket accessories or poor previous repairs splicing into the bus wiring and creating a bridge
Damaged or improperly seated connector pins allowing the terminals to touch inside the housing
Rodent damage to the harness insulation exposing and shorting the twisted pair

How to diagnose and fix it

Connect a scan tool and record all stored codes across every module to confirm which bus segment is affected and whether other modules are offline
Measure resistance between CAN High and CAN Low at the diagnostic link connector with the battery disconnected; a reading far below the normal 60 ohms indicates a short
Inspect the harness routing along the medium-speed bus for chafing, pinch points, corrosion, or rodent damage, especially near connectors and body pass-throughs
Disconnect modules one at a time from the bus and recheck resistance to isolate whether a module or the wiring is causing the short
Repair or replace the damaged section of harness using proper splice techniques and recheck bus resistance before reconnecting power
Clear codes and perform a full network scan to verify all modules communicate and the code does not return after a test drive

What the shorted bus reading really means

A resistance reading near zero ohms between CAN High and CAN Low confirms a hard short, but an intermittent code often leaves normal resistance at the connector, so wiggle-testing the harness while watching live data is essential.
The medium-speed bus often carries body and comfort modules rather than powertrain data, so a vehicle can run and drive perfectly while the code is active, which leads many owners to ignore it until a safety system fails.
A single failed module can pull the entire bus down, so technicians who replace the module that first stopped communicating may be replacing a victim rather than the cause of the short.
Aftermarket alarms, remote starters, and dash cameras tapped into the wrong wire are a frequent source of bus shorts that owners never connect to the warning light.
The code name says shorted to bus, but the same code can set from a short to ground or power on one line that drags the differential signal out of range, so voltage checks on each line to chassis ground are part of a complete diagnosis.

Can I keep driving with U0018?

If the vehicle starts, runs, and stops normally, you can drive it cautiously to a repair shop, but avoid long trips and be prepared for warning lights or accessory failures. If the engine will not start or safety systems such as anti-lock brakes are disabled, do not drive and have the vehicle towed.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.