Hi there - my name is Ian and am from Mallorytown, Ontario - I own a 2002 Chev Blazer LS ZR2 and it has almost 300K Km on it - My question is I am getting a P0155 code and I know that its a bad 02 Sensor Bank 2 Sensor 1 - my question is this, is it an Upstream or a Downstream Sensor - I have been searching but can't find a good answer - everything I've seen leads me to believe it might be the Upstream one but not sure - please help