Recently had to deal with this issue:
While at a stop light, the IMA system completely stopped functioning while on auto-stop. The IMA light came on, and the system was totally dead. The gas engine still worked fine.
I tried replacing the 12V battery, no effect.
I knew my hybrid battery was still good, because it was only a year old.
Codes reported were P1575 and 1600.
First shop diagnosed it as a failed IMA control computer, the one behind the back seat right in the middle.
I got a junkyard computer and replaced it. Same problem, no change.
I took it to Honda, who eventually diagnosed it as a failed 100 AMP main fuse.
They replaced it, and the problem was fixed.
Apparently, the fuse is inside the hybrid battery case. If you have this problem, maybe you can replace that fuse yourself and save some cash. Honda charged me about $400 to diagnose and repair. Not bad considering their repair comes with a warranty. Doing it yourself would probably cost less than $100.
I've posted a pic of the location on an Insight, taken from an Insight forum about a similar repair, and a pic of the fuse itself.

 

Where is it on the civic?

 

It is located in the hybrid system behind the back seat. According to the mechanic at Honda, this fuse is inside the hybrid battery case. You would need to remove the back seat, remove the hybrid battery, and open up the case.
I posted a picture of the hybrid battery from the Insight, with the location of the fuse inside the battery circled in red. I didn't actually replace the fuse myself, so I don't have any pictures of me opening up the hybrid battery or the fuse's exact location inside the battery of the Civic (but I assume it should look at least similar to the Insight battery).

 

OK thanks.

 

Thanks, This will come in handy in the future!