bad gas milage p420 code

My 2000 suburban has a 5.7l engine and 180k miles. It has recently started throwing the p420 code consistantly. I also sometimes get the bank 1 lean code. How should I go about remedying this? The exhaust system is in good shape with no leaks. The motor runs fine, just bad gas milage.

Check this link for the p0420 issue http://www.troublecodes.net/articles/catfailure/.

My 2000 suburban has a 5.7l engine and 180k miles. It has recently started throwing the p420 code consistantly. I also sometimes get the bank 1 lean code. How should I go about remedying this? The exhaust system is in good shape with no leaks. The motor runs fine, just bad gas milage.

Follow the link Maxwedge gave you also if you are getting a lean code you need to diagnose that first...fuel trim faults will kill a cat

I cleaned the maf sensor because I read that the oil from the k&n filter can gum it up. The truck went 40-50 miles and now has the p0171 and p0174 codes, but the p0420 has not returned. I will change the fuel filter today( it has 70K on it). I added a pint of seafoam to the tank yesterday and also looked over the vacuum lines, but did not find anything. If this does not fix it, what should I look at next.
P.S. i looked at the o2 readings and the upstream sensors both fluctuate between .1 and .9 volts at idle while the downstream ones remain mostly flat.

You are in a lean condition here, many issues can cause this including the maf, low fuel pressure, unmetered air entering the engine dirty injectors, a misfire, check this link, you will need Adobe. http://service.gm.com/gmspo/mode6/pdf/2000/2000file15_8.pdf

I looked at that link, but I am not sure what any of that meant. Anywhere else to look, maybe a little more elementary?

As I said you can see the conditions necessary to set the code, whichkkind of eliminates those areas, stay with the maf not operating in range and the other issues mentioned, also check the baro readings on your scanner and make sure they correspond to your area. Post back if they don't. Take a fuel pressure reading here, the pumps in these trucks have been problematic also. Lots of areas to address this condition.