Hello again, Code 643 is water flow switch failure...
Actually it is not so much as a flow switch failure as it is water flow failure, so we check the fuses first, and make sure the pump is running.
If the pump is running, we make sure that there is no air in the system preventing water flow, if we have all the air out of the system and the pump is running, then the problem is likely a bad flow switch.
Tell me if the water heater is being used for domestic hot water only, or if it is also being used to heat the home, and if that is the case, tell me if it is in the floor radiant heat, baseboards or radiators.. we can discuss how air bleed the system after that.
It may be however that just a fuse is blown and the pump is not running.
Let me know about all that, we can go from there.