error codes help

My service engine soon light comes on after about 10 or 15 minutes of driving. I had it scanned at Advance Auto parts,

1999 K-Series
T=5.3L
With Air Pump

Set DTC's (codes)
History DTC PO430
Catalyst Effeciency
Below Threshold
(Bank 2)
MIL Requested
Since IGN not run
Since Clear P/F

Can anyone help determine what is causing the error. O2 sensors (pre or post) or could it be the catalytic converter?