I got a 96 blazer throwing a P0420 code. I recently had a full tune up (injectors cleaned new distributor cap new plugs off the top of my head) My car is a bit rough at idle. Tomorrow I am going to test the fuel pressure and for exhaust leaks.

Which o2 sensors should be checked/replaced? What else should be looked into?

 

I would try replacing the left upstream 02 sensor (the one before the catalytic converter) first... That most likely will be your issue. alot cheaper than the next alternative, replacing the catalytic converter. if yours is California Emissions setup like mine, then the cost is well over $1000

 

99.9999999999999999% chance is the cat....po420 is usually the cat. nothing to fix it. replace it.

 

I agree with chris015. P0420 is catalyst efficiency. the computer cannot calculate efficiency with a bad O2. so if your computer calculated efficiency, it is not working and must be replaced.