Did you correct the probe's display value using a dew point meter (or thin-film carbon determination) method? If this is related, the issue is not with the oxygen probe. If repeated tests still show incorrect results (error greater than 15%), the following troubleshooting steps should be taken. Check and confirm that the compensation wire of the thermocouple and the signal wire of the oxygen probe are clean and secure, and that the connections to the instrument's corresponding terminals are correct.