I went and had my 06 Cobalt checked out because the dreeded check engine light came one and I got the P0420 threshold bank 1 code. My mechanic, who has been a good friend of mine for quite some time, told me I needed to replace my B1S1 sensor from what the diagnostics showed. He even showed me on the machine it was not flunctuating, or even being read, like it normally should. So after having the part replaced and having it checked out agian on diagnostics machine everything checked out fine. So I drove my car around to reset my computer and my check engine light came on after +-100 miles. So I went back and had it checked again and I still was getting the P0420 threshold bank 1 code. So I called the auto parts store to check if I was given the right sensor and they confermed I was. So what else could be causing my car to still get the code even after everything checked out fine in the beginning?
If the O2 sensor is replaced then the code could also be caused by a bad engine coolant temp sens. (Don't ask me how, fun thing about Cobalts). Before you buy that, also look for manifold and exhaust leaks.
The code P0420 is "Catalyst Efficiency below threshhold" First check the exhaust gases on a emissions gas tester. If the CO2 and CO together are less than around 14.5% with the engine in good condition, no faults- then you are looking at a defective cat.
Which O2 sensor did they change out? You wrote he changed out B1/S1 which would be the one pre-cat or the first one (upstream) which actually adapts the fuel to running conditions. The one which should have been looked at is B1/S2, or the one which is downstream ... the second one which its only job is to check how the cat is doing (mainly to check to make sure the cat is still in place). If you replace this and you still get the same p0420 code again, it's time to replace the cat.