Loosing eng power limp mode
Sounds like a firmware issue in the onboard computer. When you make it to a dealer with the Factory provided diagnostics, you may want them to re-flash your computer. I also agree with previous responses - once you shut the engine off, the possibility exists that the error code will not remain in the system. However - it should. You may also want to have them check the onboard low voltage regulator on the board. If it is not able to maintain a charge to the battery on the board, this may be why you are not retaining error codes.
Good luck. I'd love to hear the final diagnoses.
99 Suburban 3/4 454 4.10
34 FKWSS V-Lite - (soon to be proud owner)
Flower Mound, TX