I never worked on a Taurus, but here are few general hints:
1. I am guessing that your vehicle is from the early 90's. Alway post the
year and mileage of your vehicle, if you want advice.
2. Most KOER tests require you to tap the brake, turn the steering wheel,
and push the OD button on the transmission (if yours is an automatic with
overdrive). Seems that your codes 536 and 521 showed up simply because you
did not do those things.
3. Throwing parts at a problem that you don't understand rarely helps. The
trouble codes give you a general direction, but almost never pinpoint the
defective part. There simply is no substitute to reading a good manual,
understanding how the system that you are attempting to repair works, and
equipping yourself with at least the basic tools for the job (a DVM and a
hand-held vacuum pump if you are working on an EGR).
As far as your EGR problem goes, the EGR valve is a mechanical,
vacuum-operated device, which controls the flow of a small portion of
exhaust gas into the intake manifold when conditions are right. The computer
manipulates the opening of the valve by applying vacuum through a solenoid
(or regulator) in response to the EGR flow, which it measures through some
sort of a pressure sensor. The whole system is relatively simple, but as you
can see, has several components, all of which can fail, clog, leak, or
disconnect. Ford used at least 3 different types of EGR systems over the
years, and one of them may have an electronic control module in addition to
what I described. I believe that if you post the model year, someone on this
group will know what exactly you have, but otherwise it's hard to give any
If I may risk a guess, the pressure sensor (either a PFE or a DPFE) is the
most fragile part, because it has electronic parts subjected to the constant
barrage of hot exhaust. I would, however, advise you once again to do
methodical troubleshooting, and not follow my (or anybody else's) guess.