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Discussion Starter #1
True to the very spirit of these vehicles, on the day that I am preparing for Insight Fest 2019, I hop in the car and it doesn't start. OBDIIC&C showed P1644, but no other IMA DTCs. IMA light was not on. I have a very crispy 12v battery setup, (I very crudely replaced my 12v battery with IMA sticks) so I'm barely able to start the car, but it gets going.

So, no IMA assist or auto-stop or anything. OBDIIC&C has all dollar signs and crazy symbols for the IMA information it's completely whack.

P1644 is a failure for the ECM to read the MOTFSA signal from the MCM (Pin A16, red/blue wire) which then like disables the IMA system.

Thinking it could be an error with my wire harness I swapped it with a known good one.
No effect.

I swapped the MCM with a known good one.
No effect.

I now begin to suspect the ECM and//or a wiring fault (corrosion perhaps?) on a connector between C505 and the ECM. This would take a while to troubleshoot though.

I then think, well why not try an Arduino bypass? (I had one laying around.)
No effect. Also, the Arduino didn't turn on.

I figured the Arduino or its power supply was bad, so I pack the car, disconnect the BCM, cut the white-green wire, and head on my way to Ohio.

9 hours and two bump-starts later (thanks mudder!), with an hour to go, it comes to me:

The MCM was dead and never outputted anything on MOTFSA because it had no power. The MCM gets its +12v power from the same source as the Arduino, which explained the Arduino not powering on either. Fuse 24 (IMA, 7.5A) in the driver's footwell was blown. Putting a new fuse in and reconnecting the BCM got everything working again.

Clues that someone can follow:
  • DTC P1644
  • IMA light is OFF
  • No other IMA DTCs present
  • IMA battery gauge 0 bars
I think if you see this exact combination of stuff it means no power to the MCM. The MCM has to turn the IMA light on, but since the MCM was dead it couldn't do that. Check the fuse and that's probably where your problem lies.
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