Navien Error Code E786: Header Thermistor Failure
Warning: Your Navien boiler has detected a header thermistor failure, which could lead to system damage if not addressed promptly.
How to Fix Navien Code E786
- Step 1: Turn off your boiler at the main power switch to prevent further operation until the issue is resolved.
- Step 2: Inspect the wiring harness connected to the header thermistor for any signs of damage or corrosion.
- Step 3: If you're comfortable doing so, check the thermistor's resistance using a multimeter. A faulty thermistor will typically read either extremely high or low. If you're unsure about any of these steps, contact a professional HVAC technician.
How to Reset Navien Code E786
- 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 E786 Mean?
The E786 error on your Navien boiler indicates a failure in the header thermistor, a sensor that monitors the temperature of the boiler's water jacket. This error is typically caused by a faulty thermistor or a poor electrical connection. If ignored, it could lead to overheating and potential damage to your boiler.
Common Causes
- Faulty header thermistor
- Poor electrical connection at the thermistor
- Corrosion or damage to the thermistor's wiring
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| Boiler is displaying E786 error | Measure the resistance of the header thermistor using a multimeter | Around 10,000 ohms at 25°C (77°F), with a linear change in resistance as temperature changes |
| Thermistor resistance is significantly higher or lower than expected | Inspect the thermistor's wiring for any signs of damage or corrosion | Intact and clean wiring with no visible damage |
Applicable Navien Models
Source Documentation
Referenced in 6 official Navien technical manuals.
| Manual | Page | Reason | Action |
|---|---|---|---|
| NFB-H Installation & Operation Manual | 116 | Improper Front Panel Detected | Check for correct panel version. |
| Navien NFC-H Installation & Operation Manual | 128 | Improper front panel | Alarm |
| Navien NCB-H Installation & Operation Manual | 130 | Improper Front Panel | Contact technical support at 1-800-519-8794. |
| Navien NFB-H Installation & Operation Manual | 116 | Improper Front Panel Detected | Check for correct panel version. |
| Navien NHB-H Installation & Operation Manual | 119 | Improper Front Panel Detected | Check for correct panel version. |
| Navien NPF Installation & Operation Manual | 128 | Abnormal front control panel connection | This error occurs when a different type of a front control panel is connected to the furnace. ● Check if the front control panel connected to the furnace is appropriate for the NPF Series furnace. |
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Header Thermistor | 9015110000 | — | Find Part |
Related Codes on Same Models
Other codes that affect the same Navien model families as E786.
| Code | Description | Severity |
|---|---|---|
| E001 | Ignition Failure | Warning |
| E003 | Abnormal Ignition | Warning |
| E004 | False Flame Detection | Warning |
| E031 | Inlet Thermistor Open/Short | Warning |
| E046 | Outlet Thermistor Open/Short | Warning |
| E110 | Exhaust Blocked | Warning |
| E205 | Water Flow Sensor Failure | Warning |
| E517 | Water Flow Switch Failure | Warning |
| E736 | CH Return Thermistor Failure | Warning |
| E740 | CH Buffer Tank Thermistor | Warning |