Navien Error Code E740: CH Buffer Tank Thermistor
How to Fix Navien Code E740
- Step 1: Turn the boiler off using the power switch on the front panel, then wait 30 seconds and turn it back on. If the error reappears immediately or after a short run, move to step 2.
- Step 2: Check that all wire connections to the buffer tank sensor are secure and not visibly damaged. Do not open the boiler casing. If you see bare wires or corrosion at the connector, contact a professional.
- Step 3: Call a qualified Navien service technician to replace the buffer tank thermistor or repair the wiring. This is not a DIY repair because it requires opening the boiler and testing with a multimeter.
How to Reset Navien Code E740
- Turn off the unit using the power button on the front panel.
- Turn off the gas supply valve.
- Turn off the circuit breaker supplying power to the unit. Wait 30 seconds.
- Restore power at the breaker, then open the gas valve.
- Turn the unit back on. If the error returns, the underlying issue needs repair.
Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.
What Does Navien Error Code E740 Mean?
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.
Common Causes
- Failed CH buffer tank thermistor (open or short circuit)
- Damaged or corroded wiring between thermistor and main control board
- Loose or disconnected connector at the buffer tank sensor
- Faulty main control board (rare; only after ruling out thermistor and wiring)
- Incorrect thermistor type installed during previous service
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| E740 appears immediately after power-on | Measure resistance across the buffer tank thermistor terminals (at the sensor, not at the board) | Resistance should be between 10k ohms at 25°C to 1k ohm at 80°C. If reading is OL (open) or near 0 ohms (short), thermistor is faulty. |
| Thermistor resistance is normal at the sensor but error persists | Check continuity of wiring from sensor connector to the control board connector | Less than 2 ohms for each wire, no short between wires or to ground. |
| Wiring and thermistor check out but E740 continues | Inspect the control board connector for bent pins or corrosion | All pins straight, no corrosion. If present, clean or replace connector. |
Applicable Navien Models
Source Documentation
Referenced in 6 official Navien technical manuals.
| Manual | Page | Reason | Action |
|---|---|---|---|
| NFB-H Installation & Operation Manual | 116 | Outdoor Sensor Not Detected | Check sensor connections. |
| Navien NCB-H Installation & Operation Manual | 130 | Outdoor Sensor Not Detected | ● Ensure that the outdoor reset curve is configured properly. ● Check the outdoor temperature sensor wiring connection. |
| Navien NFB-H Installation & Operation Manual | 116 | Outdoor Sensor Not Detected | Check sensor connections. |
| Navien NFC-H Installation & Operation Manual | 128 | Abnormal operation: outdoor temperature sensor (appears only when the outdoor reset curve is enabled). | Alarm |
| Navien NHB-H Installation & Operation Manual | 119 | Outdoor Sensor Not Detected | Check sensor connections. |
| Navien NPF Installation & Operation Manual | 128 | Abnormal outdoor temperature sensor operation | This error occurs when the outdoor temperature sensor does not operate properly. ● Turn off the furnace, turn it on again, and check if the furnace is operating properly. ● If this error occurs continuously, contact Technical Support at 1-800-519-8794. |
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| CH Buffer Tank Thermistor (Temperature Sensor) | 30025909A | — | Find Part |
| Buffer Tank Thermistor Wiring Harness | — | — | — |
| Main Control Board (PCB) | — | — | — |
Related Codes on Same Models
Other codes that affect the same Navien model families as E740.
| Code | Description | Severity |
|---|---|---|
| E001 | Ignition Failure | Warning |
| E003 | Abnormal Ignition | Warning |
| E004 | False Flame Detection | Warning |
| E012 | Flame Loss | Warning |
| E016 | Overheating | Warning |
| E030 | Exhaust Overheat Sensor | Warning |
| E031 | Inlet Thermistor Open/Short | Warning |
| E046 | Outlet Thermistor Open/Short | Warning |
| E047 | DHW Thermistor Open/Short | Warning |
| E060 | Fan Motor Abnormality | Warning |