According to this list, we're #5:
https://www.worldatlas.com/articles/...f-america.html
Today, I got a new trouble code (P1116). That appears to refer to the thermos bottle temperature. And I'm guessing it's related to the P1121, since the control valve controls coolant flow through the thermos bottle. So either the wiring connection on the control valve is bad, or the valve itself is bad. Anyway, I have a new control valve. So we'll have to get that installed soon.