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.