Two days ago I was driving my 05 HCH1 and was presented with the IMA and check engine lights.
This was at the end of a 200-mile trip, with no problems with the system or gauges.

These two lights have come on once before, in 2015, which prompted me to buy a Hybrid Automotive grid charger.
(https://electricvehicleforums.com/fo...95/#post255630)

Pulled codes (and Googled meaning):
P1600 (2x) ENG - Integrated Motor Assist System Malfunction
P1433 (2x) HYB - Battery Module Deterioriation / Hybrid battery pack – deterioration

Until this, I've had no problems with recharge or discharge, and gauges have been displaying properly.
Checked the 12v battery (<1 year old, fully-charged); it tested fine.
Am on the original hybrid battery pack, and have been using the grid charger for the past 6 years with no issues.
I did not get the suggested Honda IMA software update, as I read that too many users experienced a decrease in MPG afterward.
My average has been about 44-45 MPG.

Here's what I tried:
I disconnected the 12v battery, and pulled the IMA fuse (no damage).

Grid charged IMA battery overnight. Unhooked it, and reconnected the 12v battery, replaced IMA fuse with new one.
Started and drove car, no IMA or check engine lights so far.

Scanned for, and pulled codes:
P1601 - ENG - Integrated Motor Assist System Malfunction Without Malfunction Indicator Lamp
P1444 - High Voltage short circuit.

Question: is it time to replace the hybrid battery?

If so, what is suggested: buy the complete pack, or just replace the battery sticks?

Which company is considered the best?

I know the car is 16 years old, and I had considered buying a newer one before this issue, but the used car market prices have gone up higher than I expected. Aside from the IMA issue, I haven't had any major issues with the car.

 

P1444 is common when you turn on the ignition with the grid charger active. Disconnect the 12V for two minutes to clear codes.

P1433 is a battery failure, but you're clearly limping it along with a grid charger. I'd continue on that path until it no longer works.

 

Update: I took a 200-ish mile road trip this weekend thru the steep winding mountain roads of the Great Smokies.
No warning lights came back on, and I averaged 52 MPG on the trip.
This was without hooking up the grid charger beforehand.
I wonder how much longer I can go without the warning lights returning.

 

Once you get a P1433, the packs days are numbered. If you see a battery recalibration, it's time for another grid charge. Even without recalibrations, a grid charge every 3-6 months might be warranted.

Never grid charge without active battery cooling.