Leaded fuel, an engine that burns oil... Not sure of ALL the things that can cause problems but those are sure high on the list.
There are 2. One in front of catalytic convertor and one behind the catalytic convertor.
No
No, it must be replaced.
After the catalytic convertor. Sensor one is before the convertor.
the catalytic convertor
Your best bet would be to take loose the catalytic convertor and see if it is stopped up. you can run it like that for a few mins. to see if that is what is causing the problem. it might kick on a trouble code. I had a car do that ran fine then when it warmed up a little wouldn't go over 20mph. pulled catalytic convertor off and it ran fine. If that is the problem you can replace it with a generic catalytic convertor not one from the dealer to save some money.
My Chilton repair manual engine wiring schematic, for the 1996 Ford Explorer with a 4.0 L OHV engine shows 3 . Two are upstream (before the catalytic convertor) and one downstream (after the catalytic convertor - which is an oxygen sensor but is used to monitor the efficiency of the catalytic convertor)
half way down the exhaust
Catalytic Convertor
Bad port on the spyder injector , or 0xygen sensor going bad , and pluging up catalytic convertor
Bad catalytic convertor(s) is a possibility.
On the 3.0 L and 4.0 L there are 2 oxygen sensors at the front and 1 oxygen sensor (which monitors your catalytic convertor efficiency) at the back after the exhaust gasses have passed through the convertor. On the four cylinder engine - 1 oxygen sensor at the front - 1 oxygen sensor at the back (downstream of catalytic convertor - monitors catalytic convertor efficiency) Helpfull