2003 avalanche idling/starting issues

Not sure where to start. Have only had this vehicle a couple months and love it. Know it needs work just not sure where to start. It's up at 315,000 km but I know ones that go well over 400. I replaced all 4 o2 sensors as they were all showing codes. Now just bank 1 sensor 1 is throwing a code. I'm assuming that is the drivers front one?? In the summer it ran good. Now that we are heading into cooler damp weather it's running like the old carburetor gm's but I don't have the same parts to adjust?. When idling it likes to idle at 500rpm or less. Has never stalled but sounds a bit like a diesel. Wondering do I attack spark or fuel issues on these. Thx!!

I would replace the spark plugs and wires first. Then check the fuel pressure and clean the MAF.

If you had a code for the O2 sensors there is a good chance you cooked the catalytic converters

Along with the above mentioned, I would pull the throttle body and give it a good cleaning with some throttle body cleaner and a clean shop rag.

When I first got my 2004, it idled very poorly and was hard to start.

I pulled the throttle body and gave it a good cleaning, front and back, and instantly it started better and idled perfectly.

I have done this on several vehicles we own.

These GM throttle bodies are BAD to get real nasty on the back side, causing the butterfly plate to not seat properly along with other issues.

Some have mentioned that you shouldn't move the butterfly plate when cleaning, and you can get it all pretty clean without doing so.

But I had already cleaned mine before I had ever seen anything that said I shouldn't.

I have always moved the butterfly plate to it's full range many times while cleaning, with no ill effects, so far.

I have just done so very slowly and with care to not allow the plate to snap around when I release it.

I have cleaned this throttle body and several others I own many times and have never had an issue with fully opening and closing the butterfly plate.

But your mileage may vary.

Proceed with care.

Thx. Have cleaned the throttle body. Was pretty carboned on the back side. The plugs were changed with platinum plugs about a month before I got it. Done a ton of reading on plugs and there are two types of metal How do you know which ones are best. I had on old Pontiac that I ran with a different plug number. It was a bit hotter and did better on the highway than the regular factory plugs that were to be in it. 

There was an issue with the intake manifold gaskets on these up to at least 03 that may be at least part of your problem.  On my 03, starting a few years ago, when cold started, it would leak air into the engine so that the MAF wouldn't read it, but still idle high, causing the learning function to crank way back on what it expected the MAF to read when idling correctly.  Then almost like clockwork, the warming up heads would cause the gasket to seal again.  Then when the engine returned to an idle situation it would close the throttle so much to get the MAF reading where it was when the engine was seeing extra air from the leaking gaskets, but now would be practically chocking itself, until the learning function would start to adjust the other way.  Changing the gasket isn't too bad of a job and just about any replacement you can get today has been "fixed" to prevent this from re-occurring.

So just a quick update and thanks for all the help. She's still not to 100% yet... but we now have all 4 o2 sensors reading now. Thankfully bank 1 had no power and we went in search....may I recommend all 03 owners check their electrical where the clamps are on the motor...the big bundle. Over the years the tape wrap stretched and got hard and has rubbed and destroyed at least 8 wires. (We are searching for more)


We are now in search of lean codes all around so guessing air somewhere. Thought I'd shar the wires though as me and the boy were shocked at the find. I know these had wiring issues but that's ridiculous. ?
If I knew how to post a pic I'd share the find. Thx all