As far as I know the lambda is active at all engine speeds, finetuning the mixture for optimum fuel consumption and emissions. Has also got something (not too rich mixtures) to do about prolonging the life of the cat, I read somewhere in a previous discussion.
If it is any help, my lambda always seemed to be fine when connected to the "Examiner" of my mechanics (3 times). The problem persisted and I took the chance of replacing the lambda anyway. Since then it has been running okay.