2000 Pontiac Montana O2 sensor failing and tranny slipping

Every vehicle issue that has multiple problems always begins with two steps. Fix the engine then the transmission. Engine running problems will often interfere with transmission shift points and feel, so they are always corrected first. This is the technical answer.

In my expert opinion unless you are experiencing a stalling, cutting out or major symptom, that the 02 sensors are not dramatically causing the transmission problems you are experiencing. If the shifts you are feeling are like delays between gears going up and down then you very likely have an internal problem; leaking clutch packs, sticking valve body, solenoid, etc.

To the question of aftermarket additives. They are not recommended professionally because their viscosity (thickness of flow) does not meet the standards required for the vehicle. That is the technical answer. I have had a handful of cars respond to the lucas transmission treatment for conditions like yours. It doesn't not work for hard or no shifts.

Good luck.

How much history do you have on the vehicle? Have you owned it since new and if not do you know the maintenence history?
At 177k on the odometer, if the transmission has not been serviced regulary I would not recommend the flush, a traditional filter change may be better. At that high of mileage without regular service, flushing will most likely not help your problem, it will just let you know how bad the trans really is. The trans has clutch plates with and abraisive material bonded to them, this allows the plates to "stick" together to carry the torque through to the wheels. As the plates wear the abraisive materials become suspended in the fluid, when you flush the old fluid out the abraisive materials go away too and the trans slips worse.
That being said, the vibration after it's warm leads me to believe it is the transmission not the oxygen sensor. Your vehicle should go into closed loop much faster than 15 to 20 minutes of driving.
This body style has a history of shift soleniod fairure, but that should leave a code. You stated that it only shows on O2 code, Which one is it?
I agree with Knowyourauto's opinion, engine problems will effect trans performance and the oxygen sensor is most likely not your the major fault.
All that being said, let me know what the codes have been and we will try to help you diagnose this.
Dan

Jjteddy,
Danauthier has summed it up nicely so all I have are one comment and some additional follow up questions. I found your previous post about the Montana that says you owned the van since 173,000 miles, which would mean that the history is probably sketchy. I would concur with NOT flushing the transmission and recommend not servicing the transmission in any way for the same reasons already stated.

1) What speeds are the vibrations and what gears does it seem to affect?
2) Would you also describe the vibrations between gears as a shudder, similar to going over those rumble strips on freeway exit ramps, only less severe?

3)Sorry to be redundant, but the slip...this is when the transmission either drops out of gear and the engine revs slightly, or when the trans shifts gears it takes more than one full second. Is this what you are experiencing?

Looking forward to hearing back.

Regards,

Regarding your EGR code situation, this only interferes with engine at an idle in the way of running rough or stalling but goes away as soon as the gas pedal is pushed.

The symptoms of your transmission problem are pointed to the stator part of your torque converter. It commonly seems more like a sound between gears than the engine revving or flaring.

Both of these require looking at the data/information screen on a vehicle scanner and comparing the actual values to what the expected values are. Both are also on the expensive side of repairs. EGR valve replacement is around $300 and a torque converter is much higher around $700 or more.

If you can get access to a full automotive scantool, I can tell you the speeds and values to monitor so we can figure this out. But it takes some practice using them and the freeze frame operations, so it may require you to have a full diagnostic done. In which case check online for a reputable shop that specializes in transmissions and engine diagnostics.

Please post back if you would like additional information or find a solution to the problem.

Good luck.