Peugeot 206 RC/GTI 180 P0137 Fault: O2 Sensor Replacement Not Resolving Code

Hi everyone, I own a Peugeot 206 RC/GTI 180 (registration plate year 2006). A few weeks ago, the Malfunction Indicator Lamp (MIL) turned on, and after checking with an OBD-II scanner, I found fault code P0137. I replaced both the catalytic converter and the second oxygen sensor as they were suspected to be faulty. After clearing the code and driving approximately 50 km, the MIL came back with the same P0137 code. Interestingly, every time the MIL activates, I've been pushing the car to high speeds — first 100 km/h, then up to 130–170 km/h. After replacing both components, fuel consumption slightly decreased, which makes me suspect that the issue might not be mechanical but rather related to sensor signal or system calibration. I've confirmed that the oxygen sensor is original Bosch and installed correctly. The voltage readings at both the lambda sensor and ECU side are within normal range using a voltmeter. Could it be possible that the previous owner modified the engine software (remapped) and this is causing incorrect readings or triggering the P013-7 code? Any insights, experiences, or suggestions on how to diagnose or resolve this issue would be greatly appreciated.