Takagi Error Code 312: Outlet 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 problem with the outlet temperature sensor. This will prevent hot water from being produced until the issue is resolved.

How to Fix Takagi Code 312

  1. Step 1: Reset the unit by turning off the power at the breaker for 30 seconds, then turn it back on. This may clear a temporary error.
  2. Step 2: Check the display for the error code again. If it reappears, the problem persists.
  3. Step 3: Contact a qualified HVAC technician to diagnose and repair the issue. Do not attempt to disassemble the unit yourself.

How to Reset Takagi Code 312

  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 312 Mean?

The Takagi 312 error code indicates a problem with the outlet temperature sensor. This sensor monitors the temperature of the water as it leaves the heat exchanger. The control board uses this reading to modulate the burner and maintain a consistent output temperature. If the control board doesn't receive a valid signal from this sensor, it will shut down the unit as a safety measure. This prevents the unit from overheating or delivering water at an unsafe temperature. A faulty sensor, wiring issues, or even a problem with the main control board can trigger this error. Because the unit has locked out, you will not have hot water until the problem is resolved.

Common Causes

  • Faulty outlet temperature sensor
  • Loose or corroded wiring connections to the outlet temperature sensor
  • Wiring harness damage between the sensor and the main control board
  • Sediment or scale buildup on the sensor, affecting its accuracy
  • Main control board failure

Diagnostic Steps

If... Check Expected
Unit displays error code 312 Measure the resistance of the outlet temperature sensor at the control board connector. Resistance should match the temperature/resistance chart in the Takagi service manual. Typically around 10-12k ohms at room temperature.
Outlet temperature sensor resistance is out of range Inspect the wiring harness and connector for damage or corrosion. Wiring and connector are in good condition with no visible damage.
Wiring and connector appear OK Check the voltage from the control board to the sensor. Typically 5V DC. Refer to the service manual for the exact voltage.
Voltage is present and wiring is good, but resistance is still incorrect Replace the outlet temperature sensor. New sensor resolves the error.

Applicable Takagi Models

Replacement Parts

Part OEM Number Est. Cost Where to Buy
Outlet Temperature Sensor TK190047 — Find Part
Main Control Board — — —

Related Codes on Same Models

Other codes that affect the same Takagi model families as 312.

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
321 Inlet Temp Sensor Warning
391 Exhaust Temp Sensor Warning
510 Gas Solenoid Valve Critical
610 Fan Motor Fault Warning