PO420 and PO171 codes bring questions

[Linc97]

1997 Conti is running smooth as glass. Idles like new and accels with power. I've replaced 3 Oxygen Sensors but I am getting conflicting advice from various garages including Ford. Ford says Cat Convertor is bad, a different garage has told me to replace the 4th O2 sensor. I've just turned 219k on the car.
PO420 and PO 171 codes come up after approx. 600 miles of driving. I hate looking down and seeing that SES light on the dash. HELP ME!

 

[MichiganTeddyBear]

1997 Conti is running smooth as glass. Idles like new and accels with power. I've replaced 3 Oxygen Sensors but I am getting conflicting advice from various garages including Ford. Ford says Cat Convertor is bad, a different garage has told me to replace the 4th O2 sensor. I've just turned 219k on the car.
PO420 and PO 171 codes come up after approx. 600 miles of driving. I hate looking down and seeing that SES light on the dash. HELP ME!

I would replace the 4th o2 sensor to start. its probably gonna be cheaper than replacing a cat or 2.

besides, if there is 200k on the o2's, they probably need replacing anyway.