Toyota Repair: 97 4 runner check engine light, sensors actuators, electrical circuits

Question
I have a 1997 4 runner with 170,300 miles. The "check engine" light came on when it went to 170,200 and then off when I hit 170,300. The same thing happened a few months back- probably at the 165,000 mark. It is running great - always has.
Could this just be a preprogrammed light? (I had a chev. once that had check engines come on when time for various maintenance.)
Appreciate any insight.

Answer
Hi Julie,

The check engine light is illuminated when something is faulty in your electrical circuits, sensors, actuators and more importantly the smog components. Any time the light comes on  for any reason, a diagnostic code is stored in your ECU (the cars "brain") as a soft code or a hard code. A soft code means that the light has come on at some point in the past and then turned off. A hard code indicates that there is a continuous problem and the check engine light does not shut off unless the codes in the ECU are erased by disconnecting the negative battery terminal or main fuse for 60 seconds or more. In the case of a hard code, it needs to be diagnosed right away to prevent further damage. The only way to diagnose this problem is to connect a diagnostic electronic scan tool used by dealerships and repair shops.

Thanks!