I am helpin out a gal taking full time care of her 96 yr old mom, some friends came up with a 03 GP 3100 some will pay for parts, I have the time and a warm garage to do the work, so your help, it will be appreciated by her and her friends, an me for sure. No back ground known on car, not sure what works and what doesn't yet. starts, runs, and drives that'* a good starting point. got 7 codes though.
P0420
P0113
P0134
P0135
P0403
P0405
P0420 again, on the scan twice?
I am busy replacing the fender, car was in a hit an run after a snow storm, so working on that then will go for this stuff, any help on what the fixes maybe will save me time figuring out what the needs of this nice little car maybe.thanks for helping.
Did pull the up side O2 sensor while it was warm, it looked in bad shape so it will be replaced

you sure its not a 3800? i would say given you have so many codes and it was in a wreck you have a wiring problem, did you check all the fuses? there are test procedures for testing the input voltage in the testing + trouble shooting procedures
https://www.gmforum.com/trouble-shooting-test-procedures-141/

Just slow snow sliding glancing fender crease.
It says 3100 on the engine and I have a */C 3800 they are not the same. I have no manual for this one still working on body then will put it on jack stands, take lots of pictures before I start checking and replacing.
I was surprised by 7 codes after driving it a couple miles to get it here and it was driven for a couple hundred miles to get it to town seemed to run, no really noticeable issues, most of the codes seem to be around the O2 sensor, I pulled it while engine was still warm and it looks like it had a rough life, from the looks of it.
Some people buy em and drive em when they can't get it to move anymore they get another one.
This engine has been spray painted silver so it may even be a used engine dropped in here, mechanically I think the engine is fine sensors and wiring along with plugs and wires, might need fixing, I saw two plug wires still buried and unused just left there got two new plugs and one new plug wire in trunk so betting I have a couple plugs that weren't changed either, so pictures are needed before I start unplugging lines and investigating.
Thanks for the links they always help chasing things down. well time to get moving around.

plugs were bad two had right gap of 60 one with stuck plug wire was 73 gap and loose enough to turn with fingers one gapped at 50 the other in the back was the other they didn't get out it was 75 gap. up front 2 at 60 alright one at 65. Plugged PVC valve, Pulled EGR it is working an clean. changed up stream 02 sensor it was crusted up pretty bad.
now no codes, not smooth at idle, bogs and hesitates when given the gas, and the exhaust stinks. Am I on the right path thinking the catalytic converter could be plugged and is the cause of the problem?
The exhaust smell is quite different. I've heard that'* a sign the cat is clogged?
I will be in Denver bone yards for a day or two up in the 60s and sunny lol then will put this on stands change oils an filters check brakes out and see what i can see from my back. also see what I can do with cat converter, not sure what else would be causing it, get the rpms up and it kind of smooths out but it doesn't feel right.
better get a movin 5 hr drive in the Ultra, lot more comfort in it then this computer chair.lol day lite fading fast .

unburned fuel from bad ignition will definitely cause the cat to be plugged. some ive seen get bad enough you can feel the heat and they start to get reddish

You can sometimes diagnose a plugged cat with a vacuum gage. At idle, normal vacuum for a V6 engine is around 12 to 16 inches Hg. After checking idle vacuum, snap the throttle to wide open and settle the engine speed back to about 2500 RPM. During this, the vacuum should initially fall to a very low level (not zero) at the initial throttle snap, and then regain a level at or somewhat above the idle reading. If it does not recover vacuum fully at or above the vacuum level at idle, an exhaust restriction may be suspected. Finally, allow the throttle to snap closed and engine speed to return to idle. The high engine speed and closed throttle plate should create a high vacuum until normal idle speed is reached.

Normal vacuum at idle but drops to near 0 and rises to lower than normal is one sign of a restriction in the exhaust system.