have you got a de cat? or it cud be that your cat is on the way out.
I put a de pipe on my car and after driving it for about 20 miles the engine light came on, the car drove fine and when i took it to the garage (Pumaspeed) they told me the the second lamber sensor was reading to high so it brings the light up to tell you your cat need replacing. tony reset the ecu but said the light would come back on and it would be ok because of want i did, altho i went back because my tb started playing up got the 60mm tb tony reset the ecu again and the light ant come back on after doing over 7000 miles. think the air/feul mix is running alot better.
best thing you can do is take it some where to read the ecu and they will tell you straight away