Let's get a few basic facts from Snoop. .. Then we can make some further deductions.
Use a proper multimeter on the battery terminals to measure the below.
What is the voltage of the 12v battery when the car is off?
What is the voltage of the 12V battery with the IGN on and engine not running?
What is the voltage of the 12V battery with the ign on and engine idling?
What is the voltage of the 12V battery with the ign on and engine at about 2.5k rpm?
Thanks
Unfortunately 99% of dealer techs know very little about the inner workings of the IMA system.
They will follow Honda workshop manual diagnostics for known codes, but when you don't have any it's pretty much game over for them.
The IMA motor itself is virtually indestructible in normal use. I have never heard of an electrically failed IMA motor in over 15 years.
A failed IMA motor with an open or short in the phases etc would also give an immediate un-resetable IMA code, so we can eliminate that from our enquiries for now.
The DC-DC can fail, but again it's incredibly reliable and well built. I have only seen two failed IMA system DC-DC converters over the years.
The voltages I have asked you to obtain above will tell us if the DC-DC is operating Y/N.
Note. Not operating is not the same as failed/broken.
The DC-DC will disable itself if it overheats and the MCM and ECM can also command the DC-DC on or off.
If it isn't operating then we have to decide if it has failed
or it has been shut down by other parts of the IMA system.