I have a check engine lite on a 2000 dodge dakota v8 engine ?

when ever the codes are cleared it stays off for about 3 or 4 hundred miles then it comes back on when the lite is off it pes smog with flying colors when its on it is an automatic fail i do not know what the codes are as i do not own this truck yet does any one have any suggestions as to a simple cure for this what would be the ramifications if i were to ignore this lite?