2002 3.4l speedo and transmission...

Hey all, first time posting i believe... This last month has been one thing after another with my car, never had a problem before... threw a code got it scanned said o2 sensors... replaced both of them still a code... read again said low efficiency in catalyst... thats the first thing i still have to take it in since the cat is under warranty, but my main concern started monday... the speedometer started to bounce down to 0 and back up when the car would shift gears, last couple of days its been real rough, seems like when i let go of the gas it slips out of gear and when i hit the pedal it downshifts... and then goes normal... I changed the fluid and filter today, still the same... I was shifting through reverse to neutral just a minute ago and it seems to make a popping sound when i go from neutral to drive at a stopped position... Anybody think they can help????

O2 sensor replacement was not necessary most likely. That reading only indicates there is a problem in the fuel/emissions system. First, it should be reset then if code returns you then search for the cause. They were just doing the job they were intended to do. Parts stores are making a fortune on these. Problem could be dirty air cleaner to mass air flow MAF sensor, dirty throttlebody, bad plug wire, plugs (cylinder miss) or even a loose/faulty gas cap in some cases.
There may be a poor electrical connection perhaps loose or dirtyor faulty VSS (vehicle speed sensor) located on the right (passenger side) of the transaxle extension housing. Look to see if fluid is leaking there as well (O-ring is replaceable) A must if sensor is removed!! Try cleaning connectors first if not loose. Hope this helps..Dave~
P.S. Always get O-rings from dealer service parts (all after market O-rings are inferior) At least that is what I have found over many years and my opinion only.

Can you tell me where those things are located? Are they accessible without removing anything???

Unfortunately I don't have access to photos of this that I can post or email. Maybe someone else in the forum may be able to do so? Pretty sure it's only accessable from below, backside of engine to the passenger side of transaxle extension housing, should be a single connector. I don't think anything needs to be removed for access.

Not sure if you ever had this car repaired for the VERY common coolant leak but if you did not (or even if you did) that could be the source of your problem. I have this same car and had the coolant leak problem that everyone says occurs with these. The leak tends to occur in the intake manifold gasket but will also happen at the gasket near your exhaust. Both gaskets are faulty in these cars. I had the gaskets replaced because my low coolant light was coming on and when I took it in they found a leak at the main gasket but they did not notice it had also leaked into my exhaust system. I had similar symptoms as you describe and thought it had to due with my transmision until we plugged it in and found out it had to due with emissions and sure enough, I had my cat checked out and it was shot. Apparently the coolant leaked in and destoryed it (broke it all up inside). This common coolant leak causes more problems then some may think. It is sad they refused to recall it. They have lost my business. I will never buy a Chevy again.

I have been very busy lately buying a house so have been putting this off, but bought new vehicle speed sensor yesterday swearing that was going to be the problem. I was on my way home today ready to replace the sensor, when my check engine light started flashing! I turned down my music and the car sounds rough! It sounds bad in the exhaust, and is shaking a lot more then its supposed to! I called the chevy dealer and theyre closed over the weekend! Any ideas what Im looking at? Im really hoping its just the cat under warranty, i received a letter in the mail does anyone know if i need that, because i can not find it anywhere!
Any input is appreciated, as I am clueless!

Ok a quick update, I replaced the vehicle speed sensor, and it did nothing, so i took it in to the dealer asking them to check the cat. they scanned the codes and seen it was showing low voltage at the speed sensor but noticed it had just been replaced. I guess they then followed the trail and came to the PCM, Now i had a cone filter installed a while back and it was resting on the PCM, but after time the steel netting on the filter had rubbed on the wires, and shorting a few of the wires... I wrapped everything back up and it runs fine now, although the check engine light just popped back up. Im thinking the cat is bad and they didnt even check it out thinking wiring was the problem to all. Anyone agree?