get a very good multimeter and read the voltage of the O2 sensor while the engine is idling and at normal operating temperature.
You can take readings from the diagnosis box in the engine room from its pin marked O2.
It should fluctuate between 0.3 and 0.8mv consistently and should immediately respond to throttle input. If not then the sensor is either dead, blind or lazy. replace it with a NEW GENUINE DENSO part (no old scrap) and test again. It should be normalized now. Also clean the injectors and replace the main fuel filter before doing any final judgement.