The other day a check engine light came on and went away later. It felt exactally the same as usual. The engine code was 99-9. It was listed as unknown. Can anyone, please, either look this up for me or tell me where to look it up. Thanks.
Well its not even close to a valid OBDII code. I'm assuming your jumping the DLC in some fashion to get the Honda "blink" code equivalent. Which is a large overstatement (equivalent). Their almost worthless for troubleshooting. MIL blink codes are only 1 or 2 digit. The only thing close would be a 90 which is the Evap system - non specific. Blink codes can't get as specific as their OBDII counterparts which would be either P1456, P1457 in this case.
P1456 is the "loose" gas cap code. But it can be any vapor leak in the fuel tank area.
P1457 is most commonly related to leaks in the canister area or a leaking purge / vent control solenoid.
BIG difference in where and what to begin to look for. I'd get the more accurate OBDII code before proceeding.
AFAIK the only place Honda blink codes are published is in the Factory Service Manual in parentheses next to their OBDII counterparts.