An E4 error code indicates that you have an issue with your water temperature sensor. This is the sensor that allows your water heater to display the current temperature of the water inside of it. This error code will typically be permanently displayed and your water heater will not run.
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First, unplug your water heater and verify the connection to the water temperature sensor. This can be done by disconnecting and reconnecting it several times. The sensor is located inside the water heater, at the bottom left, on your outgoing water line. The comprehensive troubleshooting guide has pictures of this sensor and its connections.