The plumber said a special repeat flush was a good bet. After a while, a realized the flush hadn't improved anything, but it was an intermittent problem. I could use the system for a few weeks, turning the hot water on and off until it ignitied. Finally, the ignition rate turned to zero. The diagnosis of needing to be flushed had been made by my home appliance insurance contracted plumber, based on the fact that I had not flushed the system. My appliance insurance denied my claim, based on the lack of maintenance, then not renewed my policy after 22 years. However, the service manual does not specify that the system needs to be flushed, only that lime in the heat exchanger would void the warranty on that part, and that if scaling occurred it should be cleaned. Just like a coffee maker.. I have a top of the line water softener system. I was not shown any evidence of scaling. I called the manufacturer, who was able to identify the correct diagnostic procedure for the computer error code, and sent me the manual with the procedure. The problem is either a bad switch or a bad circuit board. The switch can be set by hand, but requires a partial disassembly to access it. Setting it by hand, it will either work, establishing the switch at fault, or not work, establishing the circuit board at fault. When I called the plumber (repeatedly) regarding this procedure, and that I had the manual pages, he never followed up with me. This is the reason for the F in punctuality, although the first time he came out as scheduled.