U0039 indicates an open circuit condition on Vehicle Communication Bus B, meaning the data line between control modules on that bus is interrupted. This prevents modules on Bus B from exchanging information, often setting multiple communication-related DTCs across different systems.
A broken wire, cut harness, or corrosion-induced high resistance creates an open on the CAN high or CAN low line of Bus B.
Loose, corroded, or bent pins at any Bus B connector — especially at the gateway module or splice points — interrupt the signal path.
A missing, open, or incorrect-value termination resistor at either end of Bus B prevents proper signal transmission.
One module on Bus B has an internal CAN transceiver failure that pulls the bus lines to an open state.
A shorted wire to voltage or ground can mimic an open by collapsing the differential signal, though this often sets additional codes.
Rare; the gateway or PCM/ECM internal Bus B driver circuit fails, but only after wiring, connectors, and other modules are ruled out.
Almost never. The vast majority of U0039 codes are caused by wiring opens, connector issues, or a failed module on the bus. Replace the PCM/ECM only after all other causes are conclusively eliminated.
Bus B is a secondary communication network (often CAN) used for body, chassis, or infotainment modules, separate from the primary powertrain Bus A.
It depends on which modules are offline. Loss of safety-related modules (ABS, airbag, stability control) makes the vehicle unsafe; loss of convenience features may be drivable but should be repaired promptly.
With ignition off and battery disconnected, measure resistance between CAN high and CAN low at the DLC or a Bus B connector. A reading near 60 ohms indicates both 120-ohm resistors are present; 120 ohms indicates one missing; infinite resistance indicates an open circuit or both missing.