I opened the case and looked for any signs of distress or faulty components. Nothing looked suspect.

My suspicion would be the small child-boards that are soldered to the main board. These have an extensive amount of surface mount components. I've had some experience w/ surface mount resistors coming loose on a clock circuit board in a Subaru. I was able to repair but the Honda ECU boards have an extensive number of components, and none appear to be failing. On Subaru clock board you could see the failed solder jts and some actually had fallen off.

regards

Thanks for your postings TexasHonda. Wanted to chime in on my experience. I had replaced the ECU before on my Accord (had some faulty caps), and after a few years of driving, Code 21 had appeared.
I went ahead and replaced the VTEC solenoid (cleaning the gaskets, new gaskets, new solenoid), but none of that fixed the issue. Figured the check engine light that came on wasn't a big deal. But since then the check engine light had always stayed on (ignition in the run position, light doesn't go away after the blub test).
Well because the check engine light was on, it wouldn't pass smog, So I figured after reading your posts and few years later, I went ahead and replaced the ECU with a remanufactured one off Ebay (weren't cheap, close to $200, The first ECU I bought off Ebay was used at ~$50). After a replacement, no lights, seems like that solved the issue.

I did end up opening up the 2nd ECU and it looks like near the bottom left, there was a burnt spot but no immediate sign of a dead cap. That may have caused the issue...