Richmond tankless heater displays E6 and water temperature is unstable or unit shuts down intermittently.

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

• Locate heat exchanger sensor: Remove the front cover. The sensor is typically clipped or screwed to the heat exchanger body.
• Check wiring harness: Inspect the sensor wires for chafing, burns, or loose connectors. Reseat the plug at both the sensor and control board ends.
• Descale the heat exchanger: Connect hoses to the hot and cold service valves and circulate a descaling solution with a pump for the time recommended by Richmond. Flush thoroughly with clean water. Scale can cause hot spots that trigger sensor errors.
• Test sensor: With power OFF, disconnect the sensor and measure resistance. Compare to the manufacturer's chart. Replace the heat exchanger temperature sensor if out of spec.
• Overheat cutoff check: Some models include a manual reset high-limit switch on the heat exchanger. If tripped, reset according to the manual. If it trips repeatedly, a technician should evaluate combustion and water flow.