I got what I hope is not a counterfeit replacement part for U101 from an eBay seller. I replaced it, and now I am getting error #604.
Is there a way to verify that U101 is functioning correctly?
I saw another post here that this was caused by a bad U201, U102, and some zeners. It looks like the previous owner already replaced U201, and checking the voltages on its pins, it seems to be working.
I notice that the voltage across CR203 is 4.58V. Schematic says this is a 5.1V zener. I know the zener voltage does vary with current, but that seems pretty far off.