Hi

Did you MOT this in house ?

Worth double checking the MOT specific limits, occasionally Lambda limits are extended to accommodate specif vehicle characteristics.

It does sound like a surplus oxygen issue, have you confirmed gas tester is correct and free of leaks?

Lastly make sure the probe is well in try running emission check with slight restriction in tail pipe i.e. wrap a rag round the probe. If it a large tail pipe pressure pulses can pull external air into tale pipe area.

Aidan