Make sure temperature is set to 120 degrees or below.
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If there is no error code and the unit does not come on when the HOT water is turned on. This means that the flow meter is not sending a signal to the computer which starts the machine. 1st 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, 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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2nd check the wire connections; follow the wires to where it connects to the computer board. If both were connected you will want to 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.
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If the display shows two squares rotating up and down. This pertains to the 45H only. 1st push the power button. 2nd If the unit has been in operation for a while and all of the sudden the squares begin rotating. This means power was lost somehow while unit was running. 3rd The flow meter wire is loose on the computer, or the connector where the two wires connect between the computer and the flow meter are loose. 4th The remote itself may have a short, or the power cord going to the computer one of the prongs may not be making a good connection threw the connector or the computer.
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Regardless of what the error code is; take the two front screws out of the computer board and it will slide out. Pull the inserts that are lined up along the front of that board and put them back in one by one. Plug the unit back in and see if this solves any issues.