I have a 1998 ford windstar and i need upstream & downstream 02 sensor but i dont know if i need 1 or 2 upstream sensors.

Assueming it has the v-6 in it look at the exhaust pipe comming off the side of the engine there will be two up stream 02 sensors which are heated and one downstream past catalitic conveter. If may have the second style with the Y cross over with 1 upstream before the cat and one after. The difference between the 0-2 sensors is the upstrean are heated so it goes into closed loop faster. I find that on most of these the up stream 02 sensor heaters have failed. You can go to a Autozone and they can pull the codes for you and tell you which side it is that needs to be replaced. However 02 sensors get old after time and they start shifting slowly, if it shifts slowly then it's called a lazy 02 sensor. And will send a code to the computer, if you get the trouble code of the 02 sensor heater is bad you will have to change it, I recommend Replacing both. The reason is if one is going bad the other will soon follow. You can buy a socket wrench For the 02 sensors for 12 dollars makes it real easy to replace the 02 sensors when they are COLD. Hope this helps and thanks for using fix-ya.