The CEL on my 2001 Accord came on in June and stayed on despite my following Honda manual directions to tighten the gas cap and restarting and driving over a period of several days for way over 3 times! My mechanic checked it and it showed the code P0128 (thermostat range/Performance) though I wasn't having any other problems with the car at that time, and no other gauges, idiot lights, etc. showed a problem. So he reset it. A week or so after that I left for a weeks vacation and I left my trunk open this whole time so came home to a dead battery. Replaced the battery but 24 hours later my car wouldn't start again. We replaced the clamp to the negative battery post because it was badly corroded and that solved the problem. Probably didn't need the battery but because it was 5 years old I kept it. The battery issues are doubtfully related, but wanted to give you all the info. 6 weeks later the CEL comes on again and the code showed up this time as D00F05H/13635333 - Undocumented Code.

What's your opinion of the problem and solution?

 

What kind of connector was used for the negative battery terminal? Can you measure the voltage across the battery posts with and without the engine running? Then measure the voltage across the connections to the battery with and without the engine running.

I'm not sure what that code means. You may want to find a different code reader to see if it can recognize the code.