Damaged or chafed wiring on the Bus J CAN high or CAN low circuits, often near connectors or flex points; Corroded, loose, or water-intruded connectors on the Bus J network; Failed control module that is pulling the bus voltage down or not transmitting; Shorted or open termination resistor on the Bus J network; Aftermarket accessory improperly tapped into the CAN bus wiring; Internal fault in a module causing excessive bus traffic or voltage disruption

DTC codes / U00xx · Network wiring and bus faults / U007C

U007CControl Module Communication Bus J Off

Q: What does code U007C mean?

Control Module Communication Bus J Off. U007C indicates that a control module has lost communication on the J-1939 data bus, often referred to as Bus J, which is a high-speed network used for powertrain and chassis modules. The module that set the code detected that the bus voltage or message traffic on that specific CAN (Controller Area Network) line fell outside the expected range, preventing it from exchanging data with other modules.

U007C indicates that a control module has lost communication on the J-1939 data bus, often referred to as Bus J, which is a high-speed network used for powertrain and chassis modules. The module that set the code detected that the bus voltage or message traffic on that specific CAN (Controller Area Network) line fell outside the expected range, preventing it from exchanging data with other modules.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaNetwork wiring and bus faults

Symptoms you may notice

Illumination of the check engine light or a dedicated communication bus warning light
Multiple dashboard warning lights (ABS, traction control, airbag) due to shared bus dependence
Intermittent or complete failure of one or more electronic systems such as stability control or transmission shifting
Engine may enter limp mode or stall if the affected bus carries critical powertrain data
No noticeable drivability issues in some cases, with only the warning light as evidence
Scan tool may fail to communicate with certain modules or show 'no response' for those modules

Common causes, most likely first

Damaged or chafed wiring on the Bus J CAN high or CAN low circuits, often near connectors or flex points
Corroded, loose, or water-intruded connectors on the Bus J network
Failed control module that is pulling the bus voltage down or not transmitting
Shorted or open termination resistor on the Bus J network
Aftermarket accessory improperly tapped into the CAN bus wiring
Internal fault in a module causing excessive bus traffic or voltage disruption

How to diagnose and fix it

Use an OBD-II scan tool to read all stored codes and identify which modules are online and which are missing from the Bus J network
Inspect the Bus J wiring and connectors for physical damage, corrosion, or loose pins, starting at the module that set the code and working along the harness
Measure resistance across the CAN high and CAN low circuits at the diagnostic connector to check for proper termination (typically 60 ohms) and detect opens or shorts
Disconnect suspect modules one at a time to see if bus communication is restored, isolating a module that is pulling the bus down
Check for aftermarket devices or recent repairs that may have disturbed the Bus J wiring or added unauthorized connections
Repair or replace damaged wiring, connectors, or termination resistors as found, then clear codes and verify communication with all modules
If a module is confirmed faulty, replace and reprogram it according to manufacturer procedures, then retest the network

Bus J faults that fool even good technicians

A module that sets U007C is often not the faulty module itself; it is simply the one that noticed the bus problem, so replacing it first frequently does not fix the issue.
Water intrusion into a connector or module housing can cause intermittent Bus J faults that only appear in damp weather or after a car wash, making them hard to reproduce.
Aftermarket alarms, remote starters, or dash cams tapped into the CAN bus are a common hidden cause of communication codes on Bus J.
A single failed module can bring down the entire Bus J network, so disconnecting modules one by one is a reliable isolation technique before condemning any part.
The code can be stored historically from a low battery or jump-start event, so check for a stable power supply and clear the code before extensive diagnostics.

Can I keep driving with U007C?

If the vehicle drives normally and only the warning light is on, you can drive cautiously to a repair shop, but avoid long trips or towing. If multiple systems have failed or the engine runs poorly, stop driving and have the vehicle towed to prevent unsafe operation.

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.