Chevrolet Express 3500 engine replacement issue showing system too lean on bank 1 and bank 2 with knock sensors 1 and 2 on Chevrolet Express 3500

Some Chevrolet Express 3500 owners report experiencing "system too lean" issues on both bank 1 and bank 2 following an engine replacement, accompanied by trouble codes for knock sensors 1 and 2. These issues are often indicated by codes P0171 and P0174, which signal a lean condition in the exhaust system due to excess oxygen or insufficient fuel. Owners may find that these codes are triggered by upstream oxygen sensors detecting too much oxygen in the exhaust, which can result from disconnected hoses, vacuum leaks, or a weak fuel system. Additionally, the knock sensor codes suggest abnormal combustion linked to the lean condition affecting the air-fuel mixture. It is crucial for owners to ensure all connections, hoses, and vacuum lines are properly reconnected during engine replacement to prevent leaks that could lead to these problems. Regular checks of the fuel system pressure and cleaning the Mass Airflow Sensor (MAF) are also recommended to address these concerns effectively.