I have a 1999 S-10LS with a 4.3l,at,ac,cc.
The 0420 code keeps coming up.have replaced it twice,& last time gutted the converter.Code still comes up.
Can anyone tell me why code comes up?
Engine sputters under a load also.Have replaced plugs,wires,dist. cap & rotor.

 

gutting the cat will cause a p0420 to set. the p0420 sets when the oxygen storing capacity of the cat is low. Cheap aftermarket cats can cause this to set. the ecm compares the switching rate of the front and rear 02 sensors; when they switch at the same rate, the dtc sets.

 

But I have a friend with the same setup,a gutted cat,& he has no check engine light.
Also the gas mileage has increased.