My 05  3.0 Ru  est     as fault code P0420 -  Catalyst Systerm Efficancy Below Threshold...  Common code i hear with Subarus../ so think iam going to Replace both Senors..for starters..  but does anyone know how many wires mine might have to sencor as they can have 3-5 wires , just to save me getting it all jacked up to look.. Thanks Guys.    

I'd buy one and swap the one in the top of the system 1st as they are quite expensive, be a shame to but two when may not be needed, of the code goes then happy days, if not stick the old one back in and swap new one with the one you haven't swapped, again if goes out happy days. That way your not spending un necessarily.

If it's anything like mine, the upstream sensor is a 4-wire Denso. With VAT, the local dealer quoted £250 each to buy, extra to fit.

Import car parts is your best bet for cheaper oem sensors