Hey Guys,
I had this same problem with my 2002 GT30.
Did some research and found the DENSO 234-9010 to be the same (or pretty much the same) as the stock sensor. Subaru quoted me the part number 22641aa032 for the sensor, however the 234-9010 crosses to the 22641aa03C.
With more research I found out that the 22641aa03C is the part number for the USA legacys and foresters.
I ended up buying the DENSO 234-9010 from www.rockauto.com, It took 4 days from payment with the fedex international priority shipping. It cost $155 NZD. Banged it in as soon as it arrived and the check engine light went off immediately.
Driven about 200km on it so far and it seems allg.
ohh also, the DENSO 234-9010 has the same length wires and same connector plug as the stock part, the sensor looked identical as well. This made it really easy to install.
Alex