Takagi Error Code 391: Exhaust Temp Sensor

Severity
Warning
Type
digital display
Estimated repair cost: $100 — $250 (parts + labor)
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

  1. Step 1: Check the exhaust vent outside the house for any visible obstructions like leaves, snow, or debris. Clear any blockages.
  2. 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.
  3. 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

  1. Turn off the unit using the power button on the front panel.
  2. Turn off the gas supply valve.
  3. Turn off the circuit breaker supplying power to the unit. Wait 30 seconds.
  4. Restore power at the breaker, then open the gas valve.
  5. 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 Takagi Error Code 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.

Common Causes

  • Faulty exhaust temperature sensor
  • Obstructed or blocked venting system
  • Combustion issues leading to high exhaust temperatures
  • Wiring issues or loose connections to the exhaust temperature sensor
  • Scale buildup on the heat exchanger causing overheating

Diagnostic Steps

If... Check Expected
Unit displays error code 391 Measure the resistance of the exhaust temperature sensor at the control board connector Resistance should be within the range specified in the Takagi service manual for the current exhaust temperature. Typically around 10k ohms at room temperature.
Sensor resistance is out of range Inspect the wiring and connections to the exhaust temperature sensor for damage or loose connections Wiring and connections are intact and secure.
Venting appears obstructed Use a manometer to measure the exhaust vent pressure Vent pressure should be within the range specified in the Takagi service manual. Negative pressure is usually expected.
Vent pressure is high Visually inspect the heat exchanger for excessive scale buildup Heat exchanger should be relatively clean with no significant scale accumulation.

Applicable Takagi Models

Replacement Parts

Part OEM Number Est. Cost Where to Buy
Exhaust Temperature Sensor TK190012 — Find Part
Combustion Fan Motor — — —
Gas Valve — — —

Related Codes on Same Models

Other codes that affect the same Takagi model families as 391.

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