U0115: Lost Communication With ECM/PCM B

U0115 is a network communication code that logs when one control module on the vehicle's communication bus stops receiving expected messages from ECM/PCM B, a secondary engine or powertrain control module. Modern vehicles use a shared data bus to let modules talk to each other constantly, and when that conversation drops out, the receiving module logs a lost communication fault. The "B" designation indicates a secondary or redundant ECM/PCM rather than the primary unit.

Because this is a bus communication fault rather than a mechanical failure, the most common culprits are wiring issues rather than a failed module outright. A corroded connector, damaged bus wire, poor earth connection, or a module that has simply lost power to its circuit can all cause this code. It's worth checking the battery and charging system first, since low voltage across the bus is a surprisingly frequent trigger for U-series communication faults.

Severity is rated high, because the module that has stopped answering is a powertrain control module. The car will often keep running, but it can drop into limp mode or cut out, which is its own hazard in traffic, so this is not one to sit on. A degraded communication bus can cause intermittent faults in other systems that share the same network. Have the fault scanned with a proper bi-directional scan tool to check whether ECM/PCM B is actually present on the bus, then work from the wiring and connectors before condemning the module itself.