P0430 code in 2000 GMC Sierra - Grrrr

2000 GMC Sierra 1500 4X4 w/ the 5.3L V8, 127k miles. Keep getting a P0430 code with the SES light. Have replaced both bank 2 O2 sensors and plug wires, and had fuel system/ injectors cleaned which eliminated a slight miss in the engine. Pickup runs great but I have this annoying light that keeps coming on. Strangely, and this may be relevant, I can clear it and hit the highway and while driving at, say, 70-80 mph, it does fine. The light only comes on after I pull off the highway and allow the engine to go back to idle speed. If I clear the light and just drive around town, never going > 30-40, it doesn’t come on. Also, and I don’t know if there’s a correlation, it only has come on when temps have been <32-degrees. Then again, it hasn’t been warm long enough this month to see if it’ll do the same thing when it’s nice outside. This problem began about a month and a half ago when the fuel line froze during that really bad cold snap. When we finally got it to turn over and run, I had to replace the number 2 coil pack, replaced the wires at that time, and it’s been throwing the P0430 code light ever since. Like I said, the truck runs great, no loss of power, no misfires, good idle and the mileage is back to normal since replacing both O2 sensors. SHould I look at the simple stuff - new fuel filter, new plugs? Or, what? Not a mechanic, just a po’ boy trying to keep his ride running.

P0430 = “The catalytic converter efficiency is below threshold”. This could be a real fault with the catalytic converter; or, it could just be indication. To discover which, real or only indication, is the difficulty.
http://www.gmtruckclub.com/forum/archive/index.php/t-12784.html Maybe, this will help.

The catalyst system on that bank has low efficiency, so the catalytic converter may be failing. That is just one possibility (seems like the main one to me), but the thing won’t always last forever. You might check the #2 plug just for fun.