State tankless water heater shows E04 and shuts down; unit may overheat or water is extremely hot before shutdown.

Safety: Turn off power and gas. Allow the heater to cool completely before touching internal components.

• Check water filter: Close cold water supply to the heater. Remove and clean the inlet water filter screen located at the cold inlet connection. Rinse debris and reinstall.
• Descale heat exchanger: If the unit is in a hard water area and has not been flushed annually, connect a pump and hoses to the service valves and circulate a manufacturer-approved descaling solution through the heat exchanger for 45–60 minutes. Rinse thoroughly with clean water.
• Inspect temperature sensors: Locate the outlet thermistor on the hot water outlet of the heat exchanger. Ensure it is firmly attached and wiring is intact. Replace if physically damaged or if resistance readings are out of spec.
• Reset high limit: Some models have a manual-reset high-limit switch on the heat exchanger. Press the reset button if present. If it trips repeatedly, do not bypass it; find and correct the cause of overheating.