O2 and A/F Sensor Readings - I need an armchair diagnosis! HELP!

Originally Posted by L33T Z34

I assume the screen shots are from the Z sitting in the drive way logging idle and then revving to 2,500 RPM? Bank 2 (driver side) holds on the rich side per the screen shot. Funny your OBD2 code P0420 is for Bank 1, yet your log shows Bank 2 w/the flat line. Both banks are probably suffering rich conditions at various times.

Do you have aftermarket cats/headers/exhaust installed? Service manual gives the following for possible causes: three way cat bad (leaks), exhaust manifold leak, intake air leak, fuel injector(s) bad, spark plug(s) fouled, improper ignition timing.

Did you say you bought your Z used? Perhaps the original owner had test pipes or hi flow cats and just threw the oem cats back on w/cheap gaskets and/or that red rtv shi+.

I would go about solving this problem by doing these steps:
1) Ensure there's no intake leaks
2) Check spark plugs for fouling/change to new plugs
3) Ensure the oem cat "ring gaskets" & "exhaust front tube gaskets" are replaced w/oem new ones and the oem hardware: two studs, two nuts and the "demon bolt" are good/replaced. The demon bolt usually snaps when removed, so inspect to see what yours looks like.
4) Ensure the exhaust manifold gaskets aren't leaking
5) Testing the fuel injectors
6) ECU ignition timing has been changed (probably the least likely)

If you want the dealer to fix this, you're going to have to get it into limp mode. You should get codes that stick then.