That Vehicle has 2 oxygen sensors.
Well, honey, a 1999 Ford Ranger with a 4.0 engine typically has two oxygen sensors. One is located before the catalytic converter (upstream sensor), and the other is after the catalytic converter (downstream sensor). So, there you have it, two sensors keeping an eye on that engine of yours.
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
If it's like a 1991 Ford Explorer ( with the 4.0 liter V6 engine ) It has ( 1 ) oxygen sensor
There are 2 oxygen sensors. One before the catalyltic converter and one after the catalyltic converter.