I found a similar thing on a cheap water heater. California requires an additional sensor to ensure the heating chamber doesn't overheat. It's not common generally, and the error messages when it triggers are not that helpful. After a few years of intermittent water heater failures, I finally realized that there was this sensor that was causing all the problems. I bypassed the sensor with a 1K resistor, confirming the issue, then had a new sensor sent out under warranty. Quick swap and it's been back to normal. I never found any documentation or repair advice that even considered that the sensor might be bad, and since it was California-only, most repair guides or technical documentation didn't mention it.

It's really easy to have a sensor failure that indicates a major repair is needed, when the actual issue is the $1 sensor.