Navien Error Code E805: Abnormal supply air- temp sensor operation for lower limit

The E805 error code on your Navien tankless water heater indicates that the unit is detecting an abnormal reading from the supply water temperature sensor, specifically related to a lower limit threshold. This sensor is responsible for monitoring the temperature of the incoming cold water entering the unit. The Navien's control board uses this information to regulate the burner and maintain the desired output water temperature. If the sensor malfunctions or provides an incorrect reading, the unit may shut down as a safety measure to prevent overheating or other potential issues. Essentially, the control board is seeing a supply water temperature that is outside of the expected range. This could be due to a faulty sensor, wiring problems, or even unusually cold incoming water. Because the unit can't reliably determine the incoming water temperature, it locks out to prevent potential damage or unsafe operation. This error will prevent you from getting hot water until the issue is addressed.