My BOV800XL displays E05 and does not accept user input. I did a resistance/continuity check on the temperature sensor, where the connector meets the display panel (that's the easiest spot to get to the two wires going to the sensor). Mine read at 125k ohms of resistance. I think this is what E05 means, but I do not have a definitive answer on that. It makes sense that the device will not allow a user to use the device when the temperature sensor is faulty because it becomes very unsafe if it were to heat uncontrolled.
I also checked the thermal fuse and it was fine. I also checked a lot of the control/power circuit board, and all the variable resistors (blue capacitor looking things) were fine, the triads (they have heatsinks) were fine, all the resistors were fine, and even the capacitors. Notably, those yellow capacitors are X2 capacitors, they are in series with the power (power on one end, power on the other, no ground), so follow their chunky traces to know the flow of higher voltage for any others working on troubleshooting. I had a little trouble testing the relays and transformer, but I hear at least one relay click on when the device is first powered on.