thank Daox, i have an OBD II scanner and if the driving is not on a high way, which means the speed and motor RPM in a rapid change mode the O2 sensor readings keep up and down and the computer doesn't monitor any issues, however when driving on a high way in a steady state speed and RPM for about 10 mins the sensor readings come close to a fixed value and the computer replaces it with a pre-set value (0.5 or 0.4) and light the check engine, if you restart the engine the readings of the o2 senor return back to the normal operation (rapid up and down). that is way i think the issue is related to the computer, what do you think?