[car661]

I have had many of the 1996 Platforms over the years, Caprice, Impala SS, Roadmaster, and Fleetwoods.
I like 96 because of the OBD II System.
BUT Recently had a misfire from my 96 Fleetwood, and it said the downstream 02 On the pass side was bad.
So I replaced it, and it ran even worse, so I took out the brand new one, took my old one (which the vehicle was running well with, just a couple misfires) and wire-wheeled it VERY GENTLY, and put new Antiseize on the threads, reinstalled it, and now the vehicle will not run.
It coughs as though it wants to start, but sadly it's a no-go.
I have swapped the new and old 02 sensors back and forth twice, to see if somehow in someway I was missing something, but to no avail.
She won't actually run.
I had to step away to regain my sanity.
I have had I would say 9 of these vehicles with the identical systems, and this is the first time I have ever come across something like this.
Your feedback is MUCH appreciated.

 

[95Chwagon]

Do you have any OBDII codes to share? What's your fuel pressure at the rail?

 

[uvaldi]

The wiring to the O2 may have a short.

 

[Marky Dissod]

I like 96 because of the OBD II System.
BUT Recently had a misfire from my 96 Fleetwood, and it said the downstream 02 On the pass side was bad.

Just checking, what was this code? Any other codes come up?

So I replaced it, and it ran even worse, so I took out the brand new one, took my old one (which the vehicle was running well with, just a couple misfires) and wire-wheeled it VERY GENTLY, and put new Antiseize on the threads, reinstalled it, and now it coughs as though it wants to start, but sadly it's a no-go.
I have swapped the new and old 02 sensors back and forth twice, to see if somehow in someway I was missing something, but to no avail. She won't actually run.

The wiring to the O2 may have a short.

(Although I let my mechanic handle the details you handled yourself, I always thought O2 sensors were one of the things you might not want to apply anti-seize to? If I'm wrong, please feel free to call it out.)
Inclined to agree with uvaldi here.
You replaced it, tried another one after cleaning it up (hope the anti-seize didn't get on the sensor), THEN did an A-B-A test, and found no joy.

this is the first time I have ever come across something like this.

Usually, it's the obvious component.
Having re-tried that several times makes the 26 year old wiring that much more suspect.
Speaking of suspect wiring, did you check the fuel wiring? How about the ignition wiring?
You got misfires. Did you get codes for misfires? No other codes of any kind?

OBD2 LT1 cars are more forgiving than younger OBD2 cars for things like misfires.
In other words, it may not throw codes a younger vehicle with a 12200411 might throw.

 

[Z09B4U]

now the vehicle will not run.
It coughs as though it wants to start, but sadly it's a no-go.

She won't actually run.

The car starts in "open loop". The PCM uses temperature, throttle position and other information to get the car idling. After that it will use the O2 sensors once they are at operating temperature. A car will start and run with the O2s unplugged.

You are going to have to do some basic diagnostics.

Does it have fuel and spark?

Are the injectors being commanded to open and close? (Noid light check)

Are the sparkplugs sparking? (check with a timing light or other spark check)

As others asked: Do you have fuel pressure?

 

[stonebreaker]

The 'downstream' (after cat?) O2 won't affect how the car runs, it's only there to make sure the cat's working.

Putting anti-seize on the threads is a no-no (although I do it too) because it can kill the sensor.

I agree with everyone else, you've got other issues besides the O2 sensor. Start with the simple stuff (coil wire, fuel pressure, opti) and work your way up.

 

[MWP]

I agree with everyone else, you've got other issues besides the O2 sensor. Start with the simple stuff (coil wire, fuel pressure, opti) and work your way up.

Yep, the problem was just getting worse and the sensor had no impact. The old sensor likely will code again when you get the car running.

 

[Caddylack]

What brand is the new sensor?