nissan micra?

when i start up my nissan micra i have no or litttle acceleration it justs seems to puff engine management light is on permanently, i have had the emissions checked and they r fine. when plugged into machine it is saying mixture is lean so was advised to change lambda sensor which i did and thanks to those who answered my queries on how to do it has anybody any ideas what the problem could be as the advise to change sensor from garage has not solved the problem help it does get a bit better as it warms up buts struggles to reach 50mph