The high 02 is causing the the high lambda reading, as others have said most likely leak on exhaust or gas tester which you have checked, another problem we have encountered on odd times is air being drawn back into the tail pipe mostly when you can't get the prob in all the way. I would try running the gas test with a small bit of rag round the prob where it enters the tail pipe to stop air being drawn back in, see if this improves the reading.
If the above does not cure It is there any sign of a misfire ? normally I would expect to see high HC with misfire due to unburnt fuel, but sometimes injector faults can cause high o2 readings, Aidan