Some faults are what is called "latching ", some are classified as "non-latching", it depends on what the potential effect could be on vehicle emissions. It's a regulatory thing.
It's likely a detected lambda sensor total "failure " (disconnected) will be latching, which means that a service tool needs to be used to reset the fault light even if the actual fault goes away.
The light can go out on its own if a non-latching fault sorts itself.
Any OBD2 reader should be able to do this. They can be bought for not much more than £10 these days, or ask round any friends or neighbours who might have one. YouTube is your way to find out how to use one, it's very simple.
HTH