Now that I finally got the new plugs and wires

[LastDetail]

So about three weeks ago I replaced plugs and wires on my 01 Tahoe. My thread regarding that is just a couple topics down. Then just the other night the SES light came on, so I went to Autozone and pulled the code; it was throwing:
P0300 (random/multiple cylinder misfire)
P0420 (Catalst effeciency below threshold)
and P0420 Pd Bank 1.

Is it throwing these codes because the problem wasn`t fixed? Or do I just need to reset the computer?
I`m lost and any help would be supremely appreciated.
Thanks guys

[black bart]

Did you clear any codes after installing the wires.
After the guy at autozone pulled the codes did the light go out if it did has it came back on again. The 420 would be because of the misfire. If the engine misfires the exhaust is going to be rich with unburned gas.

[CharlesW]

What I have to say might not be what you want to hear.
Several GM engines have problems with leaking manifold gaskets and/or head gaskets. It seems to be most common in the 3.4, but also occurs in the 3.1, 3.8, and 4.3 V-6 engines. It does sometimes show up in the V-8`s as well. The fact that not as many V-8`s seem to have the problem could be because there are more V-6`s on the road.
You can have the leak without losing enough coolant to notice for quite some time. It can cause a random misfire. It can also cause some major damage if not taken care of.
Just my personal feeling, but I think the GM Dex-Cool antifreeze has been a contributing factor to the gasket failure. We have 4 GM vehicles right now and none of them have Dex-Cool in them. Even the 2004 Montana had it changed at less than 25,000 miles to hopefully avoid what I think may be a source of trouble. I`ll know better in another 100,000 miles.

Charles

[Prenderville]

I to eliminated the dex-cool in 99 grand am and 2001 monte and I believe car runs as well if not better and have no gaskets problems like many are experiencing 100% believe Rainmaker is right, it might be seeping in just enough to misfire. Good Luck

[Dozerboy]

Clogged injector try some Chevron Techron injector cleaner, but my (& would still set a CEL when under heavy acceleration. I put 40K on it after it started setting the CEL with no problems, other then clearing the code from time to time.