When you say "control board" not sure exactly what you are referring to. There is a voltage control box that contains a couple of printed circuit boards (PCBs) in addition to two solid state relays and, depending on the version, two or more capacitors. They can control the fuel shutoff valve which shuts down the fuel. The only fault as I remember it that will shut down the fuel valve and not be indicated on the panel is an overheat of the generator itself (not the diesel engine, but the synchronous generator windings). You can bypass the auto shutdown functions by providing un-interrupted 12V power to the fuel valve, but if you have a real problem like an overheated winding, it could be dangerous. Not recommended! Watch the voltage/frequency/amperage when it shuts down and see if it is a voltage control issue. You will have to be monitoring it when it does it. Also watch for a high load being added, like AC compressor or water heater coming on line. If it does it when a load happens, look for a bad capacitor or, in my case a bad solid state relay in the control box. One of mine showed melting and tested bad causing the engine to shut down when a load was added. If your tech changed the entire voltage control box, then that rules out the internals I just discussed. There are relays that could be overheating, but I don't have a schematic in front of me to give you an idea of where to look for them. As for an overheated engine, you should see a panel fault. However, remember the coolant tank is not circulated by the belt driven impeller. The impeller drives seawater. The coolant bleeding procedure is in the installation manual and an air lock could cause and overheat, but that should have shown a panel fault.