U0249 means the engine or body controller has lost data communication with the rear entertainment/control unit designated 'B'. This is a network/communication fault, not a sensor or emissions problem, so it usually affects rear audio, video, or climate/comfort controls rather than engine performance.
The module itself may have an internal fault that prevents it from powering up or responding on the data bus. This is the single most common root cause for a lost-communication code directed at one specific module.
Open, shorted, pinched, or corroded power, ground, or data circuits between the module and the main harness can drop the module off the network. Connector pin fit issues at the module are common after repairs or moisture intrusion.
A short, open, or excessive resistance on the CAN or LIN bus lines serving that module can interrupt communication. Other modules on the same bus may also set related U-codes.
Weak battery, failing alternator, loose ground, or high-resistance power feed can cause the module to reset or fail to initialize, producing intermittent or permanent communication loss.
Corrupted software, incorrect module configuration, or a failed update can prevent the module from communicating properly. Reprogramming or reconfiguring the module may resolve it.
Rarely, a central gateway or body control module may fail to relay messages to the rear entertainment module. This should only be considered if multiple unrelated modules are also non-communicative and wiring tests pass.
It is generally a convenience issue. It affects rear entertainment or comfort controls and does not typically impact engine performance, braking, or safety systems.
No. This is almost never a PCM issue. The cause is usually the rear entertainment module, its wiring, or the data bus circuit. Replacing the PCM would be extremely unlikely to fix this code.
You can, but if the module or wiring is actually faulty the code will usually return. Clearing is useful only to confirm whether the fault is intermittent.
The letter distinguishes between multiple rear entertainment or control modules in a vehicle. 'Rear B' refers to a specific unit location defined by the manufacturer, not necessarily a second-row or third-row position.