2002 Mercury Cougar Code 401

I have had the mighty cougar for a bit over a year now , shortly after buying it the CEL came on code 401. I was told it was the back pressure sensor so I replaced it.This is 11/07.Jan of 08 the light came back on , 401 again replaced it yet again.Thats how it started , every two months I was back at Auto Zone / Advanced Auto getting another sensor.This has happened ever since. Two months ago when the light came back on yet again with the 401 code I decided to try and find out why. I have had the car checked for carbon was told there was very little in it , I had it cleaned anyway. I then replaced the EGR Valve and the Back pressure senor light was off for maybe a month .I then added a can of Seafoam Motor tune up to the oil and gas .The car seems to be running better as well as idling better , I have more power than before , but I also have the 401 code again. Just today I replaced the O2 sensors on the engine the cat isnt stopped / blocked .The car is carbon free but I still get the 401 code. The EGR Valve and the latest BPS were both professionally installed in december of 08. I guess I am wanting to know what could possibly be throwing this code still? Am I overlooking something?
Also the car is still getting decent MPG like 20 in town.