2000 Mitsubishi Montero 3.0L - Check Engine Light, Loss of Power, Backfiring with P0170 Fault Code

My 2000 Mitsubishi Montero 3.0L is coming in with a persistent check engine light, significant loss of power, and backfiring during acceleration. After scanning the vehicle, I received a P0170 code indicating a rich fuel condition on bank 2. I performed a full tune-up including spark plugs, wires, fuel filter replacement, and a fuel injection service (BG's) as requested by the customer. All sensors—MAF, O2 sensors, EGR, barometric pressure, etc.—are reading within normal parameters. I conducted an extensive vacuum leak check and replaced the plenum gasket twice; all connections are secure. The fuel pressure regulator is functioning properly. The catalytic converter appears red-hot, which suggests excessive fuel delivery from injectors on bank 2. My main question: Could this be a faulty engine control module (ECM) or computer issue? All injectors have been tested and are working correctly.