An E3 Error indicates that the water heater has overheated, meaning it reached very high temperatures and shut itself down so as not to cause damage or harm.
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The reason a 45H/HI can overheat is because the temperature is set too high, the water flow is too low, or a combination of the two factors. If your temperature is set higher than 120F, set the water heater to 120F and try to operate it as usual. Lastly, if this is specific to one or two faucets only, check the filter screens in the faucets and free up your water flow as much as possible.