99 windstar O2 sensor

Can't recall for sure... there will be at least three.... possibly four. Contrry to what many commissioned partsmongers may suggest, O2 sensors do not require periodic replacement (though some will offer seat-of-the-pants anecdotes). Since the advent of the feedback type fuel managementin north amerca, I haven't replaced more than a handful of these sensors... and then, usually for heater failures - which will set the malfunction indicator lamp.

Going even further....even if there are mixture codes in the PCMs memory, it is extremely rare for the cause to be an O2 sensor....

The question bears asking... is there a deeper reason behind the question? If there is, perhaps we can shed more light on the subject for you...