hey guys my xsi is causing me a lot of problems again.

it started with hesitation in the first 2000 revs and engine management light would come on for a few seconds, so i was advised to change the MAP sensor. i changed it and everything was fine for a few weeks.

now the problem is back along with backfire in the exhaust when i down shift and stalls or hesitates at roundabouts without fail.

had a friend run a diagnostic on the ecu and said weak fuel mixture and the live sensor reading showed that lambda sensor was stuck on 700 mV.
so was told to change the labda sensor and go back if problem was still there.

changed the sensor but problem is still there. run diagnostic again same error but this time the sensor reading is stuck on 900mV.

he told me that the ECU has had it.

any help or ideas. could it be an air leak.