If you connect a diagnostic scanner and look at live data you should be able to see the lambda cycling rich/lean. This will show if the lambda is working or not.
When a lambda fails the engine will almost universally go to a rich condition (lambda<1), which does make me suspect that the measurement wasn't accurate, or you may also have a small leak in your exhaust system which will make the mixture appear to be lean as air is drawn into the system.
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