I posted earlier about a po420 code...bottom line is i need a cat converter replaced ...I have a 1996 4.9 I 6....muffler shop said if it were him all he would replace is the front cat that is between the 2 o2 sensors because that is the only one that can be "monitored" by the o2 sensors this one will run about 200 bucks the other one replaced will add 145 more bucks....any thoughts ...Thanks

 

After about 3 days of work on my truck, was throwing the same code (po420), same year and motor as yours. I tracked the problem down to a bad diaphram in the thermactor air control valve, mine is a calif truck and the valve has three 3/4" hoses and two 3/16" vacum hoses. The vacum tester pump cost my 35.00. The smog shop said it was my cat con to, but here in calif thats a dealer part only and was about 900.00 for both cats. Check that valve first because if the downstream port isnt opening the oxy sensors cant work properly and you will throw the po420. At least this is what happened to me.

Pick.