Have a 57 mk2 htp fabia. EMl keeps coming g on. Been to garage about 5x. My code reader says engine running too lean.garage code reader says mass sensor. Garage have changed mass sensor. New spark plugs and a couple of pipes in exhaust system. The light stays off several days and then back on. Any ideas?  Really want to get ot sorted as my daughter wants to use car for seicing test.Thanks lots. 

Check the brake servo vacuum hose for splits.

Check the underside as well as more visible bits, use a phone camera or fingertips.  You're looking for things like this (extreme example):

 

Thank you for taking the time to reply. Do these commonly set it off? Thanks lots 

It is a common fault, and quite an easy thing to check. 

Have you had a look? A generous helping of gaffer-tape around anything split like in my picture will be enough to test whether it is the cause of the intermittent fault light.