Make sure you have the code for the radio. You didn't say what kind of
a car it is. An easier way is to just pull the ECU's fuse, if you know
which one it is.

See http://www3.telus.net/public/johnings/faq.html#reset

sorry - I didn't mention that my car is a 98 Acura Integra.  If the light
comes on again after resetting, is it time to take it to the dealer?

An FYI from my Acura Dlr. webpage

Loose Fuel Cap Can Cause "Check Engine" Light

To prevent the MIL from coming on and storing DTC P1456 (Code 90), the fuel
fill cap must be tight. The on-board diagnostic (OBD II) system on affected
vehicles is sensitive to anything that might be considered a leak, including
a loose cap. This may be more likely to occur to some customers who tighten
the cap after refueling, and when hearing it click, back it off (thinking
that it's too tight).

Remember to tighten the cap until it clicks at least three times. It's
impossible to over-tighten the cap because it ratchets when it's tight.

On most '98 vehicles, a tightening precaution is printed right on the cap.

disconnecting the battery will not permanently erase codes on an OBDII
vehicle. if it comes back on, there's a dtc. if its a loose gas cap, its
P1456 code. it needs to be cleared with an obdII scan tool.