An E4 error code on your 45 model tankless water heater, indicates that there is an issue involving the flow meter sensor (also referred to as cold water temperature sensor.)
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This means that the flow meter is not sending a signal to the computer which starts the machine. First, to fix this you want to locate the flow meter at the bottom right on the inside of your heater. The flow meter sensor will have a white, plastic piece attached to the front of it (which is your auto water flow adjust device), with a set of different colored wires coming from the computer board attached to the top of it. The flow meter sensor has red black and yellow wires coming out of a black sensor that is screwed to the flow meter.
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Next, check the wire connections; follow the wires to where it connects to the computer board. If both were connected you will want to then remove the flow meter. Then, you will need to remove your water inlet on the outside of your heater, at the bottom right side. There is a metal clip that holds the flow meter sensor in place inside the heater. Remove this metal clip, and blow through the meter- it should make a zing sound when the little fan inside spins. Reinsert the flow meter sensor, re-attach your water inlet, and turn hot water on to see if this resolves the issue.