Richmond tankless heater displays E4 and will not maintain set temperature.

Safety first: Turn OFF power and close the cold water supply to the heater before removing any water-side components.

• Locate outlet thermistor: Remove the front cover. Find the outlet water temperature sensor mounted on or near the hot water outlet pipe inside the unit.
• Check wiring: Inspect the sensor's plug and wiring harness for loose connections, broken insulation, or corrosion. Reseat the connector firmly.
• Descale outlet area: If there is heavy mineral buildup on the outlet piping or sensor well, perform a full heat exchanger descaling using service valves and a pump with descaling solution. This can restore accurate temperature readings.
• Measure sensor resistance: With power OFF and wires disconnected, use a multimeter to measure the thermistor resistance and compare to Richmond's temperature/resistance chart. Replace the sensor if readings are out of range.
• Board check: If a known-good sensor still triggers E4, the main control board may have a failed temperature input circuit and should be replaced by a professional.