caperbility

i have a 1995 sti, took it for a mot last week as standard but with full decat exhaust,it failed on emmissions as i expected really fast idle was co .761 [fail] lambda 1.017 [pass] and hc 45 [pass] anyway i said ill bring it back with the original exhaust on it with 2 cats which i did failed again!!fast idle co .511 [fail ] lambda 1.oo7 [pass] and hc 24 [pass]. im a bit pissed off by this point as you can imagine anyway he says bring it back red hot which i did after using injector cleaner in with the petrol with high hopes and low and behold failed again co 2.221 [fail ] lambda o.999 [ pass ] and hc 26 pass. Could any of you good people throw any light on what i should do next as i dont really want to start replacing sensors cats etc ***** nilly as it were. should i have reset the ecu when changing exhausts? Any advice gratefully recieved as it is doing my head in lol.