Maybe rule #10

https://blazerforum.com/forum/forum-...-follow-28944/

 

The PCM is comparing the cycling of downstream O2 sensor to the cycling of the upstream O2 sensor, and has detected the lack of a working catalytic converter. It's something all vehicles built since 1996 are designed to do and is part of the mandated OBD2 protocol. Also know that the PCM will go into various failsafe modes, depending on the problem presented - so you may defeat any purpose imagined in taking the converter off, if the SES light is now on.

 

You could try installing spark plug non-foulers on the downstream O2 sensors, then clear the codes and try again. It worked good for me on a 03 trailblazer with a 5.3. As Lesmyer said, the PCM is looking for the upstream O2 sensors to have big rich/lean swings with a flat constant reading from the downstreams. If you use the non-fouler trick, you move the O2 sensor out of the exhaust stream which makes them look flat. The ECM will then think that the cats are there and functional and your DTC should go away...