'99 Ram 1500, 5.2l , auto , 112k miles.

CEL on , shows code p0420, Cat is only 5 months old , Magnaflow replacement. I changed the O2 sensors (both) and cleared the codes. The light is back on again. Any thoughts or cures.... Thanks.
Other than the light the truck seems to run fine. The plugs , wires and dist cap are all new.

 

Anyone?? I used the search, that's where I came up w/ O2 sensors , based on the CAT being new. Obviously not the problem though.

 

NOTE: A new Downstream O2 Sensor along with an aging Upstream O2
Sensor may cause this trouble code to set.
Review vehicle repair history.
Has the Downstream O2 Sensor been replaced without replacing the Upstream O2
Sensor?

Yes - Replace the appropriate Upstream Oxygen Sensor.

No - Replace the Catalytic Converter.

 

Sometimes the computer just doesn't like a high flow cat (if that's what you 've got)in this case, try a O2 simm and see if that doesn't fix it.