A short to ground or chafed wire on the CAN Bus B positive circuit, often from rubbing against a metal bracket or passing through a damaged grommet; A failed control module that is internally shorting the bus positive line to ground, pulling the entire network low; Corroded or loose connector pins at a module or inline harness connector, increasing resistance and causing voltage drop; Water intrusion into a connector or module housing, leading to corrosion and a partial short on the bus; A faulty termination resistor inside a module, altering bus voltage levels and triggering the low-voltage detection; Aftermarket electronics improperly tapped into the CAN bus, such as alarm systems or fleet trackers, creating an unintended load

DTC codes / U00xx · Network wiring and bus faults / U0079

U0079Vehicle Communication Bus B (+) Low

Q: What does code U0079 mean?

Vehicle Communication Bus B (+) Low. U0079 is a generic network communication code indicating that the vehicle's control modules have detected a low voltage condition on Communication Bus B, which is the positive side of a Controller Area Network (CAN) data bus. The computer sets this code when the bus voltage stays below the expected threshold for a calibrated period, typically because of a wiring fault, a failed module, or a short to ground.

U0079 is a generic network communication code indicating that the vehicle's control modules have detected a low voltage condition on Communication Bus B, which is the positive side of a Controller Area Network (CAN) data bus. The computer sets this code when the bus voltage stays below the expected threshold for a calibrated period, typically because of a wiring fault, a failed module, or a short to ground.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaNetwork wiring and bus faults

Symptoms you may notice

Illumination of the malfunction indicator lamp (MIL) or a dedicated communication warning light
Multiple unrelated warning lights on the dashboard, such as ABS, traction control, and airbag
Intermittent or complete failure of modules on the affected bus, leading to inoperative features like power windows or climate control
The engine may start and run normally, or it may crank but not start if the bus fault disables the powertrain control module
Often no symptoms besides the warning light, especially if the fault is intermittent or confined to a non-critical module

Common causes, most likely first

A short to ground or chafed wire on the CAN Bus B positive circuit, often from rubbing against a metal bracket or passing through a damaged grommet
A failed control module that is internally shorting the bus positive line to ground, pulling the entire network low
Corroded or loose connector pins at a module or inline harness connector, increasing resistance and causing voltage drop
Water intrusion into a connector or module housing, leading to corrosion and a partial short on the bus
A faulty termination resistor inside a module, altering bus voltage levels and triggering the low-voltage detection
Aftermarket electronics improperly tapped into the CAN bus, such as alarm systems or fleet trackers, creating an unintended load

How to diagnose and fix it

Connect a scan tool and record all stored codes across every module to confirm U0079 is active and identify which modules are offline
Obtain the factory wiring diagram for the affected bus and locate the CAN Bus B positive circuit, then inspect for obvious damage, chafing, or corrosion
Measure resistance from the CAN Bus B positive wire to ground with the battery disconnected; a reading below specification indicates a short that must be traced
Disconnect modules one at a time while monitoring bus voltage or resistance to isolate a module that is internally shorting the bus
Repair any damaged wiring, replace corroded connector pins, and reseal connectors to prevent water intrusion
Clear the codes and perform a road test to verify the fault is resolved and no modules remain offline

What a low CAN bus voltage really means

A low voltage on CAN Bus B does not automatically mean the bus wiring is faulty; a single module with an internal short can pull the entire bus low, so module isolation is often the fastest path to the culprit.
Technicians frequently replace the module that is easiest to access rather than the one actually causing the short, so always disconnect modules one at a time and watch for the bus voltage to recover.
Aftermarket accessories wired into the CAN bus are a common hidden cause, and the fault may only appear when the accessory is powered on or when the vehicle is in a specific operating mode.
The code can be set by a low battery or charging system problem that causes modules to boot at different voltages, so check battery health and ground connections before condemning the bus.
U0079 often accompanies other U-codes for individual modules, and the pattern of which modules are offline can point directly to the segment of the bus where the short is located.

Can I keep driving with U0079?

If the vehicle starts, runs, and drives normally with only the warning light on, you can drive it to a repair shop, but avoid long trips and do not tow a trailer. If multiple warning lights are on, the engine will not start, or safety systems like ABS and airbags are disabled, stop driving 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.