I was wondering if anybody has ever had a check engine light come on due to any gas treatment additive added to there vehicle? or stp product? I was doing it for smog that was coming up…the light was not on before… Is it possible to throw a code? especially a p0420 code for a cat convert efficiency bank 1?? Mine came in after I added it and went about 10 miles and then shopped and started driving it home and it came on… 
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Which one, was the light on before or after adding the gas additive
oops Sorry. Not was not on before
it may be a coincidence really, but I’ve also had similar incident on my old Pathfinder with 150+K miles
P0420 pretty much condemns a catalytic converter in bank#1, but it may be worth a shot to check the downstream oxygen sensor in the same bank for the lazy response time… or even replace it without diagnostics, as downstream sensors are usually cheap to compare to an upstream one and definitely MUCH cheaper than jumping into replacing catalyst
I do not think fuel additive is here to blame, most likely catalyst or sensor were on the last leg anyway
ok good to know. I am hoping that it is just the sensor and I will talk to my guy about doing that. thank you