Wait a sec - has this problem appeared since you changed the lambda sensor?
It's entirely possible it has failed again - I had a new one fail within a few weeks once, which was obvious when I removed it as the interior porcelain insulator was broken and rattled when I shook it.
If you can get under the car and remove it to have a look then I'd suggest that as a very first step - should be easy to unscrew if it was replaced recently.
Cheers
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