My cel is on. Have the P0420 code. Replaced the Bank 1, sensor 1 02 sensor in November. Which 02 sensor causes the cel light to come on? The upstream or the down stream 02 sensor? I bought 4 of the 02 extenders to try to fool the computer. So I can pass emissions testing. Need to know which 02 sensor to fool to get the cel off. This is on a 2010 Pro4X Xterra with 157,000 miles on it. Thanks