1987 check engine light

My mother has a 1987 Buick Century(4cyl), it has only 32,000 miles on it.
When driving on the highway, the CHECK ENGINE LIGHT comes on
periodically. I've had it in the garage several times, but the light seems to never come on so they can get a code. Any ideas why the light comes on??

About all GM cars of this vintage have an ODB1 diagnostic system. If the check engine light has come on recently many trouble codes are stored for a while in the computer even if the light went out. You can read the codes yourself by connecting the two pins of the diagnostic conncetor furthest on the upper right when looking at it from the driver's seat position. The connector is located under the dash in front of the driver's position. With the pins connected, turn on the ignition to the run position, but don't start the engine. The check engine light will begin flashing codes in sequence. The sequence begins with one flash a pause then two flashes - a code 12. This is the prefix and the ending code of the sequence. Each code will repeat three times. After the first three 12 codes, any stored trouble codes will be displayed three times, write these down. The codes are pretty much the same for most GM vehicles. You can get a repair manual for your model which should define the codes or find a web site like: http://www.s10blazers.com/codes.htm where they are available. See http://www.s10blazers.com for better instructions than the above.
Good luck finding what's wrong.