Help-What does it mean?

My 98 Plymouth Grand Voyager has a really stupid engine (3.3) to change the
spark plugs in. Just had to compain a little...lol. Anyways the point is I
bought a OBDII scanner to to pick up trouble codes off my vehicle and found
no codes stored from vehicle. What does it mean then when the instrument
panel that holds the a/c & heater control buttons flash while the car is
running and after about 15-20minutes it stops. Is there another test for
this? Am I overlooking something. I personally have never ask in a post
before and I was hoping that someone more experience than I on this could
help me out. Thanks
Daniel