PO420 ugggh cat or lean

ok it is a 2001 3400 grand am gt, the cat was replaced 4 years ago when i also replaced 2 injectors that were pluged. now i am getting the PO420 code bank 1. i replaced the upstream O2 sensor but nothing has changed. from talking to different mechanics i am told it could be the cat or a lean cyl, without spending $100.00 for a full diagnostic scan, what are my best options

p0420 is catylst system efficency below threshold.

Did you replace BOTH O2 sensors ?
It sounds like your Cat could be bad, or your down stream O2 sensor.

Theirs a test proceedure here for your O2's.
Tips and Maintenance (http://www.automotiveforums.com/vbulletin/forumdisplay.php?f=2148)

the down stream O2 was replaced with the cat 3 years ago, the test procedure, does it mean unplug the sensor and reconnect the plugs with bare wires that can be probed with a meter, if that is so can i just poke a pin in the wire and connect the probes

secondary question if i have to replace an injector because of a lean cyl can i buy just a plenum gasket or do i have to buy a whole upper gasket set

I would only back probe the plug in. If you perce the wire casing you can have problems down the road with corrosion on the inside of the wire(s).

To replace the injector you only need to pull the upper intake. You might get lucky and not even need a new one if it doesnt fall apart or stick to the LI.

Then you need to releive the fuel pressure, disconnect the fuel lines, and pull the entire injection unit up out of the intake.
You should also replace the "O" rings to prevent fuel from leaking and causing a fire. :2cents:

Did you find a resolve for this? I am having the same problem! Thanks

99.9% It's a blown catalytic converter, any muffler shop can do a test for free.

Thank you for the advice. I am going to make an appointment tomorrow to get it tested! :)