21.3% O2 reading not changing

[MethodPH]

My testo 300 today when commissioning a new install would only give a reading of 21.3% o2 for flue integrity, tried zeroing outside but stayed at 21.3 tried turning on and off still the same. When in the flue gas test port it would go down to what you would expect and 5.3. Anyone have any ideas why a high o2 reading? Was only calibrated in feb

 

[king of pipes]

Seems the O2 cell has failed has it been used on a bad boiler with high co /co2 ? Hard to say otherwise I have a 327 and it has happened on mine a long time ago now .

 

[Timmy D]

Has the 300 got a drain port for condense? My 330 gives off o2 readings if the port isn’t fully closed. Stumped me the first time it happened.