If this happened right after the dp install then its gotta be the sensor. Funny part about it is i broke my first 02 sensor when i did my dp. $350 dollars later i have a new one. FACK!!!!!!!!!!!!!!!!!!!! Bull crap my car only has 4k miles on it and the sensor would not come out.
http://www.protegefaq.net/tsb/01-019-07.html
Apparently dealers were doing it too, lol.