2005 HCH I, CVT, almost 53K miles (I have other Hondas) and original hybrid battery pack. I have had no problems at all from it and drive it often (work from home). My average has been about 44-55 MPG depending on cargo and driving habits. I use a Hybrid Automotive grid charger (installed 2015 when I first got P1600 and P1433) and run it about once every 2 months.

To the best of my knowledge, the ECM has NOT been re-flashed or upgraded by Honda. No other software updates have been done.

About 2 weeks ago the IMA and Check Engine Lights came on (P1600 and P1433, again) at the same time and stayed on. When I got home, I disconnected the 12v battery and pulled the IMA fuse for an hour. The lights went away; then, later came back at the same time.
FYI...
P1600 ENG - Integrated Motor Assist System Malfunction
P1433 HYB - Battery Module Deterioriation / Hybrid battery pack – deterioration

My friend rebuilt his Prius battery pack and offered to check mine out for free, using his scanning tools and load testers. So I pulled the pack and we tested each stick. All showed ranges from 7.81-7.86 volts. There was no corrosion on any contacts or cell damage.

I placed the pack back in the car and hooked up the grid charger, letting it run a good 24 hours for charging and balancing. After starting and running the car, the lights did not return until 77 miles later.

When that happened, the IMA status bar was about halfway, then dropped to about 3 bars. Around a mile later, it jumped up to almost a full charge.

My MPG hasn't suffered, nor has the car been behaving differently otherwise.

I tested the 12v battery when the car was off; it read as 12.6 volts. When started and running, it's 14.7 volts.

While the lights are on it still functions properly that I can see. If it weren't for these 2 lights, I'd never know something were wrong.

If the battery sticks test as good, what else could we look for? My friend suggested the Battery Control Module, or possibly a software glitch.

What else could we check next, before I feel resigned to buying a replacement pack?

 

Just because the battery sticks in the IMA pack have a decent voltage doesn't mean they have any capacity left. Could also have dead cells in a stick with over charged cells making up the difference.

Load testing each stick would give you a better picture. I vote you have some bum sticks in there.

 

I thought I had replied to this but was mistaken. Anyway, I'm re-pulling the sticks this week to do that again, with better equipment, and will do a few cycles of charge/discharge.