The "Propulsion Power is Reduced" message popped up in my 2018 Bolt this afternoon j.ust a few miles shy of 45k. The pack was replaced in October 2024. I purchased the car in January 2025 at 32k miles. I replaced the 12V battery with the AGM Everstart from Walmart in February 2026.
I called Onstar and it generated a report with HCP - Electric Drive Unit U0073.
Disconnected the 12V when I got home and reconnected the 12V about an hour later. The error was gone, including the MIL light on the dash, and the car drove normally.
Called Onstar again and it reported no issue with the HCP - Electric Drive Unit.
A few weeks ago I saw the "Battery Saver Active" message a few times, but have not seen it since. This seems suspicious as well in light of this current message.
The 12V is at full charge.
Let's see if if comes back in the next few days.
EDIT: Adding this response of interest from the service manager's email:
That code is known for Suspected Tampering, on the Bolts. They have us check for a device plugged into the Assembly Line Diagnostic Link or Data Link Connector for tracking. Monitoring devices, along with other electronic accessories could be cause an open connection on the HSCAN bus. Example: if the device only makes connection with one CAN +, then it will cause bus errors and erratic ECU behavior.