My 2012 went into limp mode the other day. The AA came out yesterday and cleared the fault codes (don't know what they were).The cat was OK for a couple of hours but then went again. Switching off seems to fix it for awhile but then it goes again. It seems to be making a whooshing noise when accelerating. It's been suggested the air mass sensor is faulty. Is this the one on top of the air intake pipe next to the airfilter? I've had this out and cleaned it with electrical contact cleaner but it looked brand new anyway. Could this fail after 4 years do you think? I also took the air filter out which looked a bit oily. This was only serviced in August so wouldn't have expected it to look like that but I'm not experienced so not sure if it's normal.
Any advice would be gratefully received.

A whooshing noise could be a split in the intercooler pipe - they are common and hard to detect not always throwing up the correct error code if any - plenty of earlier posts on here to help you with where and what to look for  No Longer the owner of a 2013MY Fuji White GS

"Fiat Lux" a philosophy not a car

Thanks for that. I've had a look at at few posts about limp mode but nothing to suggest what you have. I'll investigate now.

On another note, any thoughts on the air filter oily?

Could be the same - the split hose would release oily fumes which may be picked up by the air intake - worth checking but they can be REALLY hard to find when stationary - the pressure needs to build under load for the split to open up and the "whoosh" happens as pressurised air escapes from the pipe, this in turn reduces the flow of gas into the engine via the intercooler and turbo and you get power loss (and maybe a fault code).
It's all explained much more eloquently in other posts which I am sure you will find No Longer the owner of a 2013MY Fuji White GS

"Fiat Lux" a philosophy not a car