Open or shorted wiring in the Bus B CAN high or CAN low circuits, often from chafing, corrosion, or rodent damage; A failed control module on Bus B that is holding the bus lines low or high and disrupting communication for all nodes; Loose, corroded, or backed-out pins at a module connector or splice pack on the Bus B network; A defective CAN transceiver inside one of the modules on Bus B, causing the bus to go off; Water intrusion into a connector or module on Bus B, leading to intermittent or permanent bus failure; Aftermarket accessories or improper repairs that tapped into the Bus B wiring and introduced a fault; A faulty gateway module that isolates or routes Bus B traffic and has failed internally

DTC codes / U00xx · Network wiring and bus faults / U0077

U0077Vehicle Communication Bus B Off

Q: What does code U0077 mean?

Vehicle Communication Bus B Off. U0077 is a generic communication fault indicating that a control module has lost communication on Vehicle Communication Bus B, which is the second CAN (Controller Area Network) bus in a multi-bus vehicle network. The module that set the code detected that the bus is off, meaning it is not receiving the expected voltage differential or message traffic on that particular network segment.

U0077 is a generic communication fault indicating that a control module has lost communication on Vehicle Communication Bus B, which is the second CAN (Controller Area Network) bus in a multi-bus vehicle network. The module that set the code detected that the bus is off, meaning it is not receiving the expected voltage differential or message traffic on that particular network segment.

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 because several modules share the affected bus
Intermittent or complete failure of features controlled by modules on Bus B, such as body, climate, or infotainment functions
The vehicle may start and run normally if the fault is isolated to a non-powertrain bus segment
No noticeable symptoms besides the warning light when the fault is intermittent or affects only a non-critical module
A no-start condition if the affected bus is required for immobilizer or powertrain communication

Common causes, most likely first

Open or shorted wiring in the Bus B CAN high or CAN low circuits, often from chafing, corrosion, or rodent damage
A failed control module on Bus B that is holding the bus lines low or high and disrupting communication for all nodes
Loose, corroded, or backed-out pins at a module connector or splice pack on the Bus B network
A defective CAN transceiver inside one of the modules on Bus B, causing the bus to go off
Water intrusion into a connector or module on Bus B, leading to intermittent or permanent bus failure
Aftermarket accessories or improper repairs that tapped into the Bus B wiring and introduced a fault
A faulty gateway module that isolates or routes Bus B traffic and has failed internally

How to diagnose and fix it

Connect a scan tool and read all stored codes in every module to confirm which modules can and cannot communicate on Bus B
Inspect the Bus B wiring harness and connectors for visible damage, corrosion, or loose pins, starting at the modules that are offline
Measure resistance between CAN high and CAN low on Bus B with the battery disconnected; a reading near 60 ohms indicates proper termination, while a very low or very high reading points to a wiring or module fault
Check for short-to-ground or short-to-voltage on each Bus B line with a multimeter or oscilloscope while wiggling harnesses to catch intermittent faults
Disconnect modules one at a time on Bus B to see if communication is restored, which isolates a module that is pulling the bus down
Repair or replace the damaged section of harness, connector, or the faulty module identified by the previous steps
Clear codes and perform a road test to verify that Bus B communication is stable and the code does not return

What a bus-off code really tells you

U0077 does not name the module that set it, so the first step is always to check which modules are online and which are missing from the scan tool's module list.
A single failed module on Bus B can take down communication for every other module on that bus, making it look like a widespread electrical failure when the cause is one component.
Intermittent U0077 codes are often caused by a harness rubbed through near a body seam or by water entering a connector that looks dry from the outside.
Some vehicles use Bus B for body and convenience functions only, so the car may drive normally and the code may be the only clue that a module is offline.
Replacing modules before checking bus termination resistance and wiring integrity is a common and expensive misdiagnosis trap.

Can I keep driving with U0077?

If the vehicle starts, runs, and has no safety-related warnings, you can drive it to a repair shop, but avoid long trips and expect that some accessories may not work. If the engine stalls, the vehicle will not start, or multiple safety systems are disabled, 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.