Infiniti: Check Engine Soon Light

I have a '97 Q45 with 54,945 miles. The engine starts, idles, runs
and accelerates perfectly. There has been no decrease in gas mileage
and the car barely registered on a recent California Smog test. Very
recently, the "Check Engine Soon Light" has remained lit. Before I go
to the dealer and spend what will undoubtedly be a great deal of
money, is this something that is programmed to happen at a certain
mileage? If so, is there a way to reset the light so it would be
available for a true problem? I ask because years ago I owned a Volvo
whose check engine light came on about every 10,000 miles. A service
man took pity on me and told me that it was designed to do that and
how to reset the (hidden) odometer that was connected to the light.
Any info will be appreciated.

If you want to repair it yourself,
Get car scanned at one of the autoparts shop, they'll do it for free.
Error code should narrow down the cause.
Go from there.