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There is a printed circuit card inside the unit, I will point it out below, use a hair drier to warm it up to 120F or 130F but not over 140F. if you get any change in performance the printed circuit board is bad,
You will need a clamp around amp meter or a multi meter at least do diagnose the heating element cylinders. There is no specialist on those water heaters in the Escondito area that I can find.
It is not a common water heater.
Let me know how that test turns out, we can go from there without a time limit.