I have a 1997 F150 4.6 L 4WD with ~272 k miles and I am getting a periodic Check Engine light with code P0420 set (Catalyst System efficiency below threshold, Bank-1). In addition, the engine sometimes practically stalls when decelerating to a stop (RPMs go down to ~200 then bounce around before settling ~600 RPM). I did the EGR and Idle Air Control (IAC) sensor checks described in the Haynes Repair Manual (EGR Voltage, IAC Voltage/Resistance measurements) and they were all within spec. Any ideas/help would be greatly appreciated.
Thanks,
GaryB