206 GTI emisions fail - any suggestions?

[Jaguar steve]

Mrs JS's 137 bhp Gti failed it's MOT on emissions - specifically carbon monoxide content after 2nd fast idle excessive at 2.43 and Lambda reading outside specified limits at 0.930.

Basicaly it's running too rich

MOT station couldn't help to pinpoint problem to either failed Lambda sensor or catalyst and don't have deep enough diagnostics.

Going to check air filter but any other suggestions to help diagnose before I get my wallet out?

Over to you Chaps

[Minordelay]

Yep could be lambda sensor it needs plugging in to check live data and see if it's switching. Find a good independant

Nige