If you have a Tech II or real time ODB II reader, you can see the O2 sensor 1 and 2 voltages. 1 should fluctuate, 2 should stay flat.
That's a lot of driving you've done. I assume it's varied: some highway, some city, some starts, etc.
Hopefully it doesn't pop up a catalyst efficiency code after all that.
Yes, I still have the same aftermarket O2 sensor. My understanding was that for the catalytic converter really important is driving at the 55 mph on the highway and that was what i really was doing.It may be operating within spec but not within parameters to set the readiness. Anything from the signal output to the heater performance could be the issue. You do need to monitor the sensor - maybe even both sensors - to see where the issue is. Are you still running that crappy aftermarket O2 sensor?
Remember that readiness monitors are not about distance... They are about specific scenarios like idling, cold starts, acceleration, etc.
Somebody on this forum recommended GM driving cycle. Here is the linkNot sure where that came from, but that's not what I would expect. There needs to be varied driving, varied use of throttle, etc. to set the monitor. Usually the thing needed to set the cat monitor is several pulls from 20-50 and some cruising then deceleration. There's a thread recently where I discussed typical approaches to setting readiness monitors. Cruising at 55mph is PART of the requirement, but not the full requirement. And if it's been 600 miles I'd almost guarantee you'll eventually see a check engine light for something.