Takagi Error Code 321: Inlet Temp Sensor

Severity
Warning
Type
digital display
Estimated repair cost: $100 — $250 (parts + labor)
Warning: Your Takagi tankless water heater has detected a problem with its inlet water temperature sensor and has shut down to prevent potential damage. You will have no hot water until this is resolved.

How to Fix Takagi Code 321

  1. Step 1: Check the water heater's display for any other error codes. Note them down, as they may provide additional clues.
  2. Step 2: Reset the water heater by turning off the power at the breaker for 30 seconds, then turning it back on. This may clear a temporary error.
  3. Step 3: If the error persists after resetting, contact a qualified HVAC technician to diagnose and repair the issue.

How to Reset Takagi Code 321

  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 321 Mean?

The Takagi error code 321 indicates that the water heater's control board is receiving an unexpected signal from the inlet water temperature sensor. This sensor is responsible for measuring the temperature of the incoming cold water supply. The control board uses this information to properly modulate the burner and maintain a consistent output temperature. If the sensor is faulty, or the wiring is damaged, the control board won't know the incoming water temperature, and the unit will lock out for safety. This prevents the unit from overheating or operating inefficiently. It's important to address this issue promptly to restore hot water service and prevent potential damage to the unit.

Common Causes

  • Faulty inlet water temperature sensor
  • Loose or corroded wiring connections to the sensor or control board
  • Damaged wiring harness leading to the sensor
  • Sediment or scale buildup on the sensor, affecting its accuracy
  • Control board failure

Diagnostic Steps

If... Check Expected
Unit displays error code 321 Measure the resistance of the inlet water temperature sensor at the control board connector. Resistance should be within the range specified in the Takagi service manual for the measured water temperature. Consult the resistance/temperature chart.
Resistance is outside the specified range Inspect the wiring harness and connectors for damage, corrosion, or loose connections. Wiring and connectors should be clean, secure, and free from damage.
Wiring and connections are good, and sensor resistance is incorrect Replace the inlet water temperature sensor. New sensor resolves the error code.
Sensor resistance is within range, wiring is good, and error persists Check the voltage at the control board connector for the sensor. Voltage should be within the range specified in the Takagi service manual.

Applicable Takagi Models

Replacement Parts

Part OEM Number Est. Cost Where to Buy
Inlet Water Temperature Sensor TK10014 — Find Part
Control Board — — —

Related Codes on Same Models

Other codes that affect the same Takagi model families as 321.

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