OBD11 CODE P0421

2003 Hyundai Santa Fe

Computer problem
2003 Hyundai Santa Fe 4 cyl Two Wheel Drive Automatic 72000 miles

OBD Code Po421
Changed the catalytic converter recently.
Engine would hardly go over 15 mph.
Repaired broken valve #2 cylinder.
New valve job.
New rebuilt transmission.
Suspect Vacuum leak or oxygen sensor
Dale Blackwell
July 13, 2008.

Ok, so vehicle is running fine now and only need info on code?

Code Definition: P0421 - Warm Up Catalyst Efficiency Below Threshold

Jul 13, 2008.