Here's what Todd Day of Technomotive has to say on the matter:
"As far as I can tell, there are only three ways that you can see a CHECK ENGINE light, but then not be able to clock out the code.
1) The ECU for some reason loses power or otherwise crashes and does a hard reset while driving. The CHECK ENGINE light comes on for five seconds because the ECU always turns it on for five seconds as a bulb test when power is first applied.
2) The ECU flags a legitimate error. After you turn off your engine, a Mitsu shop tech sneaks into your car with a MUTII scan tool and clears the codes while you are not looking. (1991-94 only)
3) The ECU flags a legitimate error. You turn the key to ACC or OFF, wait for the relay to go *CLICK* after seven seconds. The fuse that runs your radio memory and footlights and ECU BACKUP MEMORY is burnt out and your ECU loses its mind. You turn the key to ON to clock out the codes, but everything is gone.
If you have a case of #3, try turning the key to ACC to kill the engine, but immediately bring it back to ON. This will keep the main power supply to the ECU going, and you should be able to clock the codes."