What should be done when a check engine light comes on if car is still running fine?

AnswerNot sure what year and type vehicle you have. Some vehicles (older) you were able to manually flash the codes. With the 96 and newer vehicles need a scan tool of some sort. You can purchase a OBDII Code Reader for about $100, which will able you to read and erase codes. Get one that is able to read CAN protocol which is now being used by newer vehicles. The previous OBDII protocol being used was a ISO. This will enable you to read the Generic OBDII side, NOT the OEM side. You would need a OEM compliant tool. If you would take the vehicle in, the usually cost for a diagnostic read would be $65.00, but there some car parts stores that will read the code for free, but they would like to sell you a part that might or might not fix the problem. First read the code then find out the possibilties of not fixing the problem. AnswerFound on this or another web site that disconnecting battery cable and reconecting solved this person's problem. I tried it. It has worked so far. J

I tried disconnecting the battery on my Mazda 2002 millenia. Yeah it worked, for only a short while. When the odometer registered 500 (additional miles) the check engine light re-sets and comes back on).