P0171 without any problem signs
 

I get a P0171 at least once a month. The dealer cannot detect any problem, all indications are ok. This usually happens when I accelerate very suddenly for some time and then come back to normal. At the low rpm the light will come up.

I have seen some posts about sensors or air going into gas tank but the dealer insists that if there was such a problem, I would be facing problems, but I don't. Everything works just fine.

Did they inform you of what tests they conducted? See the attached information from the shop manual. As you can see, there are a bunch of things that could cause this error code. If it continues to appear, find another dealer who will be willing to get to the bottom of the problem.

BTW, have you modified the car in any way--particularly the intake or exhaust systems?

No modification at all AFAIK, since it is an import car, but there is a small external button on the dash board that seems to have been installed after market which I don't know what it is and nobody so far managed to figure out what it could be. Its cables go deep into the dashboard but there is one small plug similar to those found in ac adaptors or chargers. Maybe its a cutout bypass?