To verify if oxygen sensors are still working well with the current calibration, you just have to read the sensor in air. Please make sure to have the right temperature on your device. If values are between 20 and 22% of oxygen, oxygen sensors are still well calibrated. If not, we recommend you to re-calibrate them or to replace them if needed. For more information please download the Application Note N°3 or contact-us to buy new sensors.