That Vehicle has 2 oxygen sensors.
According to the engine wiring schematic in my Chilton book : For a 1999 Ford Ranger , 4.0 liter EFI , V6 engine : ( there are 3 oxygen sensors )
My 1995 Ford Explorer with 4.0 L OHV has 2 oxygen sensors (Helpfull)
four
Check
My 1995 Ford Explorer with the 4.0 L OHV engine has 2 , so I'm assuming your 1995 Ford Ranger 3.0 L would also have 2
According to the 1999 Ford Ranger Owner Guide : ( the SuperCab gas tank holds 19.5 U.S. gallons )
there should be 3/1 on the left header then 2 going back
There are many causes to this problem, in a 1999 Ford Ranger 3.0L. It is usually caused by a spark plug firing when its not suppose to, or late/delayed timing.
There are three, two just below the manifolds and one after the catalytic.
For the 3.0 liter V6 engine in a 1999 Ford Ranger , with engine oil filter change : ( 4.5 quarts ) according to the Owner Guide
There are 2 oxygen sensors. One before the catalyltic converter and one after the catalyltic converter.
If it's like a 1991 Ford Explorer ( with the 4.0 liter V6 engine ) It has ( 1 ) oxygen sensor