HELP! Cats clogged?

[nhproxr]

Had problem with my Yukon this past weekend. Towed snowmobiles up north and truck had no power at all above 3000 rpm. Felt like it was tied to a telephone pole over 3 grand. Check engine light came on right away. Put scanner on it yesterday and it came up with cylinder 2 misfire (no idea why new plugs, wires, cap/rotor 15,000 miles ago) and low voltage detected on both bank 1 and 2 number 2 O2 sensors. Correct me if I'm wrong but aren't the number 2 sensors behind the cats?? Does this mean sensors are bad or cats are clogged. Truck has 133,000 miles on it and as far as I can tell cats look original. I have noticed a slight decrease in power lately even with the truck unloaded driving around.

[Butch 02 Denali]

Fairly easy to test the pressure in the exhaust before the cats. I drill and tap the pipe before the cats with a #21 drill and tap 10/32. Use a 10/32 barbed fitting and put a short length of high temp silicone hose, connected to a long enough hose to bring a 0-10 or 0-15 pressure gage into the cab. I just run it out the side and in the window, tying it up so it doesn't drag on the ground. Make a full throttle run and watch the gage. A reading of 5 lbs or less is good, between 5-10 is getting bad and you are losing allot of performance. Anything approaching 10 or over is clogged cats. A good freeflowing exhaust system will produce about 3-4 lbs of backpressure at full throttle, less that that and you will lose some bottom end torque, but gain top end HP. Most stock exhaust systems will produce about 5-7 lbs of backpressure.

After the test, replace the fitting with a short 10/32 screw.