OBD code P0421
 

I have seen some older posts about this one but have more questions. 2007 Mazda 3. Code reads "Warm-up catalyst below threshold (Bank 1). Just prior (1-2 days) to the check engine light coming on I had an injector flush. Could this be a contributor? It would appear that most discussions point to either a bad O2 sensor or a bad precat. Where to start change sensor ($200?) or precat - much more. I have been told it is OK to drive but will not pass an emission test. I would appreciate any input.

depending what they flushed it with, it could have coated the O2 sensor or cat with who knows what causing the fault to pop up. If you have driven it for a few days, it should have burned off any contaminants if that was the problem - though it could have permanently damaged something too.
There is a procedure to check the O2 sensor functionality, but you need a scan tool to read the voltages & voltage ranges.
I'd start with having the code cleared and see if it comes back, if not, don't worry about it. If it comes back on (could take a few days), complain to whoever did the injector flush and see if they have anything to say about it - they will say it's not their fault though.

O2 Sensor
 

Thanks for the info. They did use BG injector flush. When they called, as you predicted, they were sure the flush wasn't at fault. I gave up and took it to the Mazda dealer. They will do a scan which will tell if it is the sensor($$$) or precat (warranty). The code did not stay cleared after substantial driving so guess it is something "permanent".

Thanks for the input.