98 Ford Winstar GL 3.8L: replace 02 Sensors --> advice?

Where I live, vehicles have emission requirements to pass an annual State Safety inspections. My Van recently had this inspection and the shop said my van failed because the Check Engine Light on the dash was
ON. They did diagnostics and said that at least one 02 sensor was bad. At the time, I really did not want to spend the money! (That was about a month ago.....). I paid the shop $800 in other work during the same visit, and I paid a LOT for the diagnostic...
The shop did not know which sensor failed, but suggested replacing all of them.
I'd like to install the new sensors myself. I've seen one and it looks pretty easy. I do a lot of routine maint. on my motorcycles and cars.
I've seen new sensors online selling for about $20 - $35 each each.
What I could use a little help with is:
1. A diagram showing the location of all the sensors. I understand ther are 4: is that correct?
2. The acutal specs and /or part nos of the sensor I need.
3. Any advice on doing this job.
Once I get the van to pass the inspection, I want to sell it. This Ford has been a very expesive vehicle with a LOT of prob's since I bought it in 98!! It has 128,000 miles now.
Thanks very much!!
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I am not familiar with your vehicle, butI have never heard of any vehicle that had more than two. My car has one, and I paid ~ $40 locally for a Bosch aftermarket O2 sensor. And yoes you should replace all of them at that mileage. It's a wearing part.
Learn how to pull your own codes, it's easy and in the long run much cheaper than paying a mechanic.
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I think you can download the service manual here: http://www.fordcds.com /
Not sure if a van would be on any of those CD's, but it's worth a shot.
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The Windstar definitely has 4... two are upstream of the cats and two are downstream.
Additionally, this Windstar is OBD2 compliant and flash codes are not available - a scan tool or (yech) a code reader must be used. Beware of assumed knowledge.
For the original poster, I would steer clear of a shop that cannot identify which HO2S is which on their scan tool... the PIDs should be clearly marked (HO2S 1-1, HO2S 1-2, HO2S 2-1 and HO2S 2-2). You must also realize that HO2S sensors can have several different codes referring to them.... If the code is an electrical concern referencing the heater circuit, then replacing the affected sensor will *most likely* be the cure. If the code has more to do with sensor activity or fuel mixture, further diagnostics would be in order.

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Thanks, everyone for your help!
Jim,
Are these sensors fairly easy to locate and change out?
Jim Warman wrote:

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