OOPS...CHECK ENGINE ALL CODES READ "H"

[RICK'S C5]

I am a bit perplexed.......got a check engine light after standing at a downward angle with the engine running for a few minutes....won't go away. All gages look great.......under hood....no detectable problems. Fluids are all in order. The codes all read "H" for history. Any suggestions? Bad fuel? Bad O2 censor? I do notice a slight miss at idle cold.

(1998 C5 40,000 odometer)