Takagi Error Code 391: Exhaust Temp Sensor
Warning: The tankless water heater has shut down due to a potential issue with the exhaust temperature sensor. Hot water is unavailable until the problem is resolved.
How to Fix Takagi Code 391
- Step 1: Check the exhaust vent outside the house for any visible obstructions like leaves, snow, or debris. Clear any blockages.
- Step 2: Reset the unit by turning off the power at the breaker for a few minutes, then turning it back on. See if the error code clears.
- Step 3: If the error code persists after checking the vent and resetting the unit, contact a qualified HVAC technician to diagnose and repair the problem.
How to Reset Takagi Code 391
- 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.
What does 391 mean?
The Takagi error code 391 indicates a problem with the exhaust temperature sensor. This sensor monitors the temperature of the exhaust gases leaving the unit. If the temperature exceeds a set limit or the sensor malfunctions, the unit will shut down as a safety precaution. This is important because excessively high exhaust temperatures can indicate combustion problems, venting issues, or other malfunctions that could damage the unit or, in very rare cases, pose a safety risk. The unit locks out to prevent further operation until the issue is addressed. The sensor is designed to protect the unit from overheating and to ensure proper combustion.
Affected Models
Replacement Parts
| Part | OEM Number | Est. Cost |
|---|---|---|
| Exhaust Temperature Sensor | TK190012 | — |
| Combustion Fan Motor | — | — |
| Gas Valve | — | — |
Related Codes on Same Models
| Code | Description | Severity |
|---|---|---|
| 101 | Combustion Abnormality | Critical |
| 111 | Ignition Failure | Critical |
| 121 | Flame Failure | Critical |
| 200 | Overheating | Warning |
| 207 | Bypass Valve Error | Warning |
| 291 | Flow Rate Issue | Warning |
| 312 | Outlet Temp Sensor | Warning |
| 321 | Inlet Temp Sensor | Warning |
| 510 | Gas Solenoid Valve | Critical |
| 610 | Fan Motor Fault | Warning |