Which O2 Sensor????

Can someone tell me which O2 sensor code 0420 Catalyst System Efficiency Below Threshold (Bank 1) refers to?
Thanks

this is the sensor that is after the catalytic converter on the driver side. what this code means is that the converter is not getting hot enough to burn off the emmissions. after screwing around with gm for months, the bottom line was that the left side or bank 1 as they call it converter was plugged. had to replace both converters as an assembly. cost was 680.00 cdn.

this is the sensor that is after the catalytic converter on the driver side. what this code means is that the converter is not getting hot enough to burn off the emmissions. after screwing around with gm for months, the bottom line was that the left side or bank 1 as they call it converter was plugged. had to replace both converters as an assembly. cost was 680.00 cdn.

WOW! 700 hundred bones! I have the stock y-Pipe w/cats for a 2000 silvy, 5.3L Z71 ECSB in my shed. 70K miles on them, will sell them to whomever for 280 bucks shipping included. They look fine and never had any mod's on truck when they were on. Took them off for dynatech header kit (includes cats). Will take pics if you want.

How many miles are your truck? running excessively rich? mods?