i have a po420 code comin up on my 03 ram 4.7l engine . have replaced the upstream o2 sensor with a new one . a day later the code returned...very hard on fuel when the engine light is on,, milage on truck is fairly high 200k...i know cat converters are expensive,,,but has anyone run into this problem?,,,should i replace the cat?...or punch it out?

Have you changed the plugs in your truck lately? A basic tune-up might cure the problem, to much un-burnt fuel will give you an up-stream o2 code

Maybe the lack of compression in the cylinders, if the sensor gets ratio's out of its parameters, it cannot adjust the fuels trims properly, which throws the code