cutting a long story short MY96 Wrx is going to fail mot co2's up at 2.22 (-0.300)at hits highest and lambda out, but only just out at 0.94, (0.97-1.03) on idle is better but still nowhere near low enough. Nothings changed since last year other than engine servicing etc. as far as i understand the systems got two lamda's can anyone please tell me what each of them are used for as far as telling the ecu what to do if you catch my drift?
Realisticially i cant afford genuine, are there any makes i should avoid?? obviously wont be using ebay crap. il clean it up and get it cycle checked etc. short of that its a new cat which im assuming wont be cheap, any ideas where would be a good place to get one!?? any help very much appreciated ive got a week to scratch my head

many thanks. dale.