I also agree with way2old.
Very common cause of oxygen sensor codes is wiring.
Very common cause of codes along the lines of "lean bank .... " like 174 & 173 is vaccum leaks.
The most common cause of the vaccum leaks on a '00 - '03 windstar would be from leaks at the intake manifold.
Do a search on "Intake TSB", or "174" to see a whole bunch of information on the TSB.
Involves replacement of bolts in the upper intake assembly.
There is even a link to some great instructions, parts needed, and pictures....
http://leckemby.net/windstar/windstar01.html
Please post the numeric codes that you have....It would be very highly unusual for ALL 4 of them to have gone bad.
And at $50 each.....adds up...
If you do replace, you can buy the Bosch "OE" style sensor at Autozone, advance auto, etc.
Bosch makes them for FORD.
Buy a "Oxygen Sensor Socket"....which is a deep socket with a slot up the side to accomodate the wires.
Spray some "PB Blaster" onto the threads and let it soak.
You DO run the risk of breaking the threaded mounting ring out of the exhaust. This happened to me on the rear most catalytic converter.....the "downstream" sensor mounted right into the back part of the converter....and with very little force, the ring broke right out of the converter. Fortunately, I was replacing the converter assembly....and not the oxygen sensors.
The way that the sensors are set up (using my '96 for reference), there is an oxygen sensor before (upstream) and after (downstream) each converter.
The upstream sensor is used by the computer for air / fuel mixture monitoring and correction.
The downstream sensor is used by the computer to monitor the catalytic converter to make sure that it is working "catylist efficiency".
They are called "heated" oxygen sensor as each sensor has a heater inside. The sensors work by a chemical reaction. The sensor must be hot enough to work....so the heater does that.
The chemical reaction is caused by the difference in the oxygen that the sensor "sees" in the exhaust and in the outside air. This chemical reaction creates a voltage....which the computer uses to maintain the correct ratio of fuel and air.
If you take an oxygen sensor out....DO put some anti seize (high tempurature rated) on the threads.....to make is possible to take it out again in the future..... Also, if installing a new oxygen sensor.....do the same thing.