A short to ground or short to voltage on one of the two bus wires, often caused by chafed wiring or a pinched harness; An open circuit in the bus wiring, such as a broken wire or a disconnected connector, that breaks the network path; A failed control module that is internally shorting the bus and pulling the entire network down; Corroded or loose terminals at any module connector on the affected bus, creating high resistance or intermittent contact; Water intrusion into a connector or module housing that causes corrosion and signal degradation; Aftermarket accessories improperly tapped into the bus wiring, introducing noise or a short; A failed termination resistor inside one of the modules, which can disrupt bus voltage levels

DTC codes / U00xx · Network wiring and bus faults / U007B

U007BControl Module Communication Bus I Off

Q: What does code U007B mean?

Control Module Communication Bus I Off. U007B is a generic network communication code indicating that the control module responsible for monitoring or coordinating the vehicle's primary communication bus (often called Bus I) has detected that the bus is electrically off or inactive. The module sets this code when it cannot detect the normal voltage activity or message traffic on that bus segment, meaning the network is not functioning as designed.

U007B is a generic network communication code indicating that the control module responsible for monitoring or coordinating the vehicle's primary communication bus (often called Bus I) has detected that the bus is electrically off or inactive. The module sets this code when it cannot detect the normal voltage activity or message traffic on that bus segment, meaning the network is not functioning as designed.

SystemNetwork

Code typeGeneric (all makes)

SeverityHigh

Fix difficultyProfessional

AreaNetwork wiring and bus faults

Symptoms you may notice

Multiple warning lights illuminated simultaneously, including the check engine light, ABS, airbag, and traction control lamps
The engine may start and run normally, or it may crank but not start if the affected bus carries critical powertrain messages
Intermittent or complete failure of dashboard gauges, climate controls, radio, or other body functions
A no-start condition where the ignition is on but the engine does not crank or fire
Often no symptoms besides the warning light if the bus is only partially degraded and critical modules still communicate
Erratic transmission shifting or reduced power mode if the powertrain control module loses communication with other modules

Common causes, most likely first

A short to ground or short to voltage on one of the two bus wires, often caused by chafed wiring or a pinched harness
An open circuit in the bus wiring, such as a broken wire or a disconnected connector, that breaks the network path
A failed control module that is internally shorting the bus and pulling the entire network down
Corroded or loose terminals at any module connector on the affected bus, creating high resistance or intermittent contact
Water intrusion into a connector or module housing that causes corrosion and signal degradation
Aftermarket accessories improperly tapped into the bus wiring, introducing noise or a short
A failed termination resistor inside one of the modules, which can disrupt bus voltage levels

How to diagnose and fix it

Connect a scan tool and read all stored codes from every module to identify which modules are offline and which bus segment is affected
Inspect the wiring harness and connectors at each module on the affected bus for chafing, corrosion, water intrusion, or loose pins
Measure the bus voltage on both communication wires with the ignition on and compare to the manufacturer's specification, typically around 2.5 volts on a healthy CAN bus
Perform a resistance check across the bus wires with the battery disconnected to verify termination resistance, usually about 60 ohms on a CAN network
Disconnect modules one at a time to see if the bus voltage returns to normal, isolating a module that is shorting the network
Repair or replace any damaged wiring, connectors, or the faulty module identified in the previous steps
Clear all codes and perform a road test to confirm the bus remains active and no communication codes return

What most technicians miss on U007B

A single failed module can pull the entire bus offline, so disconnecting modules one by one is often faster than chasing wiring through the whole harness
The code does not always mean the bus wiring is broken; a module with an internal short can set U007B even when every wire and connector tests perfectly
Aftermarket alarms, remote starters, or phone chargers plugged into the data link connector can backfeed voltage onto the bus and cause this code intermittently
Low battery voltage or a failing alternator can cause modules to drop off the bus and set U007B without any actual wiring fault
Some vehicles require a specific wake-up signal or ignition cycle sequence before the bus activates, so testing with the key in the wrong position can mislead you

Can I keep driving with U007B?

Do not continue driving if multiple warning lights are on or the engine runs poorly, because the vehicle may enter a reduced-power mode or stall without warning. If the only symptom is a warning light and the car drives normally, you can drive cautiously to a repair shop, but avoid long trips and high-speed driving until the fault is diagnosed.

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.