Navien Error Code E740: CH Buffer Tank Thermistor

The E740 error on your Navien boiler means the control board has detected a problem with the thermistor that monitors the temperature of the central heating (CH) buffer tank. The buffer tank is a small water storage vessel used in some Navien combi-boiler installations to prevent short cycling. When the thermistor fails or its wiring is compromised, the boiler loses the ability to accurately read the buffer tank's temperature, causing the system to lock out for safety. This error is specific to Navien systems that include an external buffer tank with a temperature sensor. The boiler expects to see a specific resistance value from the thermistor based on the water temperature. If the reading is out of range—either an open circuit (very high resistance) or a short circuit (very low resistance)—the E740 code will appear. This is not a user-resolvable error in most cases; the thermistor or its wiring must be checked by a professional. Common triggers include a failed thermistor, damaged wiring between the thermistor and the control board, or a loose connection at the buffer tank sensor plug. The code will typically clear once the faulty component is replaced or the wiring is repaired, followed by a power cycle of the boiler.