"Check Engine" Light And Car's Computer Question ?

Hello:
Have been getting a "Check Engine" light on my '97 Buick Le Sabre.
Totally random. Sometimes on starting, other times while driving. Other
times not at all. No discernable difference in cars performance whether light is on or off.
If I bring in for service, Murphy being Murphy, am convinced light would probably be off for them.
Question: Once light is on, is the fault indication stored in the cars computer, so even if light is off when brought in, they can diognose what caused it ?
Might as well also ask: any thoughts on what might be cause ?
Much thanks, Bob
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The code and other data is stored in the computer in a freeze frame so the technician should be able to analyze it. As to what can cause it- anything. First thing to do is make sure the gas cap is tight and all fluids are correct level.

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