A broken or chafed CAN+ wire in the medium-speed bus harness, often at a flex point or near a connector; Corroded, pushed-back, or loose terminal in a CAN bus connector; Water intrusion into a connector or module causing an open on the CAN+ circuit; A failed module that internally opens the CAN+ line, dropping the bus for other modules; Aftermarket accessory wiring tapped into the CAN bus that cuts or bypasses the CAN+ conductor; Physical damage to the harness from rodents, collision repair, or improper routing

DTC codes / U00xx · Network wiring and bus faults / U0012

U0012Medium Speed CAN Communication Bus (+) Open

Q: What does code U0012 mean?

Medium Speed CAN Communication Bus (+) Open. U0012 means the medium-speed Controller Area Network (CAN) data bus is open on its positive (CAN+) wire, so modules that rely on this bus cannot exchange messages. The computer detects an open circuit on the CAN+ conductor, typically because voltage on that line stays outside the expected range or the bus goes silent.

U0012 means the medium-speed Controller Area Network (CAN) data bus is open on its positive (CAN+) wire, so modules that rely on this bus cannot exchange messages. The computer detects an open circuit on the CAN+ conductor, typically because voltage on that line stays outside the expected range or the bus goes silent.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaNetwork wiring and bus faults

Symptoms you may notice

Multiple warning lights on the dashboard, often including the antilock braking system (ABS), traction control, and airbag lamps
The engine may start and run normally, but some accessories or driver-assist features stop working
Intermittent or complete loss of communication with modules such as the body control module or transmission control module
A scan tool may fail to connect to certain modules or show them as 'no communication'
Often no symptoms besides the warning light when the open is on a branch that only affects non-critical modules
Erratic operation of power windows, door locks, or climate control if those modules share the affected bus segment

Common causes, most likely first

A broken or chafed CAN+ wire in the medium-speed bus harness, often at a flex point or near a connector
Corroded, pushed-back, or loose terminal in a CAN bus connector
Water intrusion into a connector or module causing an open on the CAN+ circuit
A failed module that internally opens the CAN+ line, dropping the bus for other modules
Aftermarket accessory wiring tapped into the CAN bus that cuts or bypasses the CAN+ conductor
Physical damage to the harness from rodents, collision repair, or improper routing

How to diagnose and fix it

Connect a scan tool and record all communication codes from every module to map which modules are offline and which bus segment is affected
Inspect the medium-speed CAN wiring and connectors for obvious damage, corrosion, or loose pins, starting at modules that are not communicating
Measure resistance between CAN+ and CAN- at the diagnostic link connector (DLC) with the battery disconnected; an open on CAN+ often shows an abnormally high resistance
Check voltage on CAN+ with the key on and modules awake; a normal active bus shows a fluctuating voltage around 2.5 volts, while an open may pull it to a fixed value
Perform a continuity test on the CAN+ conductor from the DLC to each module on the medium-speed bus to isolate the open segment
Repair the open circuit by replacing the damaged wire section, terminal, or connector, then clear codes and verify communication with all modules
If the wiring tests good, substitute or isolate suspect modules one at a time to see if the bus recovers

What a CAN+ open really tells you

U0012 is often set by a module that is still communicating, because it detects the open on a branch it shares with a now-silent module, so the code source is not always the failed part.
A medium-speed CAN bus is separate from the high-speed powertrain bus, so the engine may run perfectly while body or chassis modules are offline, which leads many people to ignore the code.
Aftermarket alarms, remote starters, and dash cameras are frequently tapped into the CAN+ wire and can create an open that only appears after the installation.
Resistance checks at the DLC can look normal if the open is on a stub branch rather than the main bus, so voltage and continuity tests at each module are more reliable.
A module with internal CAN+ failure can mimic a wiring open, and unplugging modules one at a time is the quickest way to prove whether the fault is in the harness or the electronics.

Can I keep driving with U0012?

The vehicle may be safe to drive if the engine and brakes feel normal, but avoid relying on driver-assist features and get the fault diagnosed soon because other modules can drop offline without warning. If multiple warning lights are on or the vehicle behaves erratically, stop driving and have it 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.