Just to clarify what you have posted in your vehicle profile data... You list your Power Plant as being the
GM 4.3L Ecotec V6. This would be an unusual swap in motor for the 2002-2009 Trailblazer and Envoy GMT360 Truck Line Series. Can you confirm that under the Hood... your engine actually looks like this one?
As for the CAT being plugged up ...Before pulling the Upstream O2 Sensor in order to investigate that possibility ...You should probably examine the Data between the Upstream Heated O2 Sensor Bank 1 Sensor 1 as possibly being
STUCK RICH using a Scan Tool that can Read Live Data. But ...if the Downstream O2 Sensor is reading
Very LEAN... then your Upstream O2 Sensor would need to be replaced. If NOT... then the check for the Pressure Relief leading to the CAT by removing the Upstream O2 Sensor should then be done to confirm the Clogged Cat as being the actual problem.
In these conditions... As soon as you take off down the street... the Bad (STUCK) Rich Condition Signal from the Upstream O2 Sensor...would signal the PCM to cut back on Fuel due to an imaginary Overly Rich Condition. The natural result would be a sudden Drop Off in Power down to the minimum acceptable level without causing the Engine to Stall. If the O2 Sensor R&R is called for... You should stick with using only the Heated O2 Sensors recommended by our other Members... and avoid Mixing rather than properly Matching the acceptable Brands... as PAIRS whenever possible. However... If the CAT proves to be The Culprit... an OEM flavor as the replacement is always preferred to any After-Market versions :