U0078 means a control module has detected that communication network 'Bus F' is disabled or unresponsive. This indicates a break, short, or severe fault on the data bus wiring, or that a module connected to that bus is disrupting the entire network. The module setting the code may not be the one actually causing the failure.
An open, short-to-power, short-to-ground, or cross-short between the Bus F CAN high and low wires is the most common cause. Corrosion, chafing, or prior collision/repair damage are typical triggers.
Loose, corroded, bent, or pushed-out pins at a junction connector or module connector on Bus F can interrupt the network. Moisture intrusion into a connector is a frequent real-world cause.
CAN buses rely on termination resistors, usually located inside two modules. A missing, open, or out-of-spec resistor can collapse bus communication and set U0078.
An internal failure in any control module connected to Bus F can hold the network lines at an abnormal voltage. Isolating modules one at a time often reveals the culprit.
The main engine or gateway computer is rarely the root cause of a bus-off code. It should only be considered after the wiring, connectors, termination, and all other modules on Bus F have been ruled out.
It is usually not recommended. A disabled communication bus can shut down safety, braking, or powertrain systems, and the vehicle may enter a limp-home or no-start condition.
Usually no. U0078 is a network fault, and the PCM is rarely the cause. Most fixes involve wiring, connectors, termination resistors, or another module on Bus F. The PCM should be suspected only after everything else is ruled out.
When the bus goes down, every module that depends on that network loses communication with the others and sets its own communication-fault codes. The other U-codes are usually effects, not separate failures.
You can clear it, but if the wiring or module fault is still present the code will generally come back immediately or soon after driving. Clearing alone does not fix the underlying bus problem.