Here you go, just found this which confirms what I said above :
"If the fault is a high lambda reading, with a high O2 reading but with normal CO and HC readings, the most common cause is post combustion air leaks. A small exhaust system air leak will leak air into the exhaust before any blows can be detected. Only a small volume of clean air introduced into the exhaust will cause a car to fail the MOT test. "
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