hi mate i had bank 1 sensor 1 and bank 2 sensor 2 go on my st170 i used to own i think sensor 2 is the one after the cat these are a common fault on st170's have a look in the other post i made at the time ive listed part numbers price etc from when i did mine you could always unplug the first sensor just to double check its the rite one if you have a fault code reader it should pop a bank 1 sensor 1 fault up hope this helps cheers mark