Tankless heater shows E5 and shuts down; water may briefly get hot then turn cold.

Safety first: Turn OFF power and close the cold water shutoff valve to the heater.

• Locate inlet sensor: Remove the front cover and find the inlet water temperature sensor on the cold water inlet manifold.
• Inspect and reseat connector: Check the thermistor plug for corrosion or looseness. Disconnect and reconnect to ensure a solid connection.
• Clean inlet filter: Many Richmond tankless models have a cold water inlet screen. Close the water valves, relieve pressure by opening a hot tap, then remove and clean the inlet filter screen of sand, rust, or debris. Reinstall securely.
• Test thermistor: With power OFF, disconnect the sensor and measure resistance with a multimeter. Compare to Richmond's spec chart. Replace the inlet temperature sensor if readings are incorrect.
• Control board diagnosis: If a new sensor does not resolve E5, the main control board may be defective and should be tested and replaced by a qualified technician.