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 low, meaning the vehicle will typically continue to operate without immediate safety risk, but you shouldn't ignore it indefinitely. 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.