2013 Honda Civic Hybrid driving with failed IIU (3rd gen hybrid, 9th gen Civic) 2012-2015

2013 Honda Civic Hybrid driving with failed IIU (3rd gen hybrid, 9th gen Civic) 2012, 2013, 2014, 2015

Disclaimer:
IF YOU CHOOSE TO DRIVE WITH SIMILAR FAILURES/WARNINGS, IT IS ENTIRELY YOUR DECISION AND YOU'RE ON YOUR OWN. I MAKE NO WARRANTY ABOUT HOW YOUR CAR WILL BEHAVE. YOUR CAR MAY LEAVE YOU STRANDED OR BURST INTO FLAMES OR START DATING A PRIUS. ANYTHING COULD HAPPEN. THE ODD BEHAVIOR WHILE DRIVING IN THIS MODE COULD LEAD TO AN ACCIDENT. IT'S UP TO YOU TO FIGURE IT OUT AND MAKE THE DECISION THAT'S RIGHT FOR YOUR SITUATION.

Corrections:
DC-DC converter 12V charging RPM range is ~1500-3500 (not 3000 as mentioned).

Warning lights / messages we observed:
Warning messages: Check IMA System. Check Charging System. Check Brake System.
Warning lights: Check Engine Light. IMA light. 12V Battery light. Brake system light. After one stop and startup, we received an airbag warning light which stayed on until the next time we shut the car down. It didn't come back after that.
Other failures: When the failure first occurred, and before I understood the importance of keeping the RPM between 1500 and 3500, the battery drained and we briefly lost power steering. That resolved itself once we started driving again and the battery charged up.

I'm posting this video after the repair was complete. The problem was the IIU (Intelligent Inverter Unit). Honda had a problem with these units failing due to bad solder joints, so they extended the warranty to 15 years or 150,000 mi, whichever comes first per Honda Service Bulletin 20-030. Our local dealership repaired the vehicle at no cost to us.

I found it interesting that the inverter unit still functioned to supply high voltage to the DC-DC converter, yet would not charge the hybrid battery. Before embarking on this long road trip, we confirmed using a voltmeter that the 12 volt battery was receiving charge above 1500 RPM. Note, the 12V battery light DOES NOT GO OFF even though the 12V battery was being charged between ~1500 and 3500 RPM. Therefore, it is not a reliable indicator of whether your 12V battery is being charged or not.

If you experience a similar failure, your car may behave differently than ours, even if the failure is also the IIU. Therefore, before travelling, I suggest it would be prudent to confirm that the 12V battery is being charged via the DC-DC converter...possibly using a voltmeter, but even better would be using a scan tool that can report voltage while driving. We saw ~13.5V being applied to the battery when between 1500 and 3500 RPM. Also, if I noticed that the hybrid battery was becoming depleted while driving, I wouldn't take it on a long trip. Also, there is some weird behavior of the transmission when operating with a failed hybrid system.

Other behaviors worth noting when operating with a disabled IMA system:
On this hybrid, normally we only have to momentarily put the key into the "start" position to start the car. It starts up very smoothly using the electric motor that is part of the IMA system. However, with the failed IIU, we had to hold the key in the "start" position. It would do nothing for about 1 second, then a backup starter cranked the engine over just like a non-hybrid car.
On the ~1000 mile trip home, we had about a dozen "thunk" events when making quick reductions in throttle setting. It was quite unsettling and can't be good for the transmission. Smooth throttle transitions were necessary to avoid these "thunks". And, we did, indeed, have to slow down going up the mountains to avoid the RPM from getting too high.
When using cruise control, going uphill would cause the RPM to increase (in order to maintain speed), but when the RPM got to about 3500, the cruise control would disengage, and it was common to have one of those transmission "thunks". Constant attention to the tachometer was necessary, even when using cruise control, to avoid these events.
The car without the IMA system has only about 90 HP. It's about 110 HP with a functioning IMA system.
Gas mileage on flat land was almost as good as normal...we got about 43 mpg driving on the of highway. Mileage went down considerably going through the mountains...we observed around 34 mpg.