It's not a reset button, it's a slide-switch, behind the dash.
It works like a 2-way light switch in a home, in conjunction with a switch at the odometer, this is to make the owner bring their vehicle in for service at certain milage points. When this is the problem, the codes cannot be read at all until the switch is flipped under the dash, then the dealer charges you lots of money for nothing, and your on your way. This may not be your problem, there are many things it could be, allthough EGR and the mil duty switch seem to be the most common. The best way to check is to get it scanned, if they can't scan it, it's the switch, otherwise you'll get a readable code.
Here's a pic(borrowed from the zuwharrie.com BBS) of the switch location, and I'll add the article too.
I believe this applies to OBD I (1995 and earlier) vehicles only...
The switch is visible in the Mirror and has been superimposed in the correct location.
At a given factory set milage your echoic engine light will turn on. This is done by the manufacture to bring the vehicle in for inspection / emissions check up.
I do not currently have the milage or all the details but I do now have a picture of the magic switch. Sometimes the location varies with in a few inches but often turning the switch to the other position will cancel the check engine light.
Bullwinkle just performed this on his 1995 4-dr 1.6L 16v.
Bullwinkle stated that his Check engine light would not blink or show any codes until the switch was flipped.