P0438 (12) Fault in Mercedes SLK 172 3.5L 2011 Model with 40,000 Miles - O2 Sensor and EGR System Inquiry

Hi everyone, My Mercedes-Benz SLK 172 (3.5L, 2011 model, 40,000 miles) has been showing P2201/14 codes intermittently for the past four years. Live data indicates an imbalance in rear NOx readings, though the vehicle drives normally with no performance issues—only requiring periodic reset of the Check Engine Light (CEL). This is a stratified-charge gasoline engine equipped with a storage-type NOx catalyst followed by a diesel-style NOx control system before the rear catalytic converter. It also features EGR or air injection downstream of the upstream catalytic converter. A few days ago, the P220-than code disappeared and was replaced by an active P0438 (12) as soon as the engine started. Live data from the upstream oxygen sensor shows normal readings. The upstream O2 sensor is a 6-wire type, and according to Mercedes-Benz documentation, it includes built-in temperature sensors—contrary to some sources on this forum. Disconnecting the upstream O2 sensor triggers a P0050 code (heater circuit failure) but does not clear the active P0438 (12) fault. I have not yet identified any obvious open or short in the wiring harness between the sensor and engine-mounted ECU, though there is no visible external damage. I'm seeking expert input from experienced Mercedes-Benz owners and technicians on whether this could be a wiring issue, sensor malfunction, or an ECU-related problem related to NOx control systems. Any insights would be greatly appreciated.