Takagi Error Code 121: Flame Failure
How to Fix Takagi Code 121
- Step 1: Ensure that the gas supply to the unit is turned on. Check the gas shut-off valve near the unit and at the gas meter.
- Step 2: Check for any obstructions around the vent termination outside the house. Clear any snow, ice, or debris.
- Step 3: Reset the unit by turning it off and then on again. If the error persists, do not continue to reset it. Call a qualified HVAC technician or plumber immediately.
How to Reset Takagi Code 121
- 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 Takagi Error Code 121 Mean?
Error code 121 on your Takagi tankless water heater indicates that the unit attempted to ignite the burner, but the flame was not properly established or was lost during operation. This is a safety feature that shuts down the unit to prevent unburnt gas from accumulating, which could lead to a dangerous situation. The system monitors the flame through a flame rod sensor. If the sensor doesn't detect a flame when it should, or if the flame is lost mid-cycle, it triggers this error. This issue prevents the unit from providing hot water and needs immediate attention.
Common Causes
- Gas supply interruption (e.g., gas valve closed, low gas pressure)
- Faulty flame rod sensor or wiring
- Blocked or partially blocked vent system restricting airflow
- Igniter failure preventing initial flame
- Malfunctioning gas valve not opening fully or at all
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| Unit displays error code 121 | Check gas supply pressure at the unit's gas valve test port while the unit attempts to fire. | Should be within the manufacturer's specified range (typically 4-7 inches of water column for natural gas, 8-11 inches for propane). |
| Gas pressure is within range | Inspect the flame rod for proper placement, cleanliness, and any signs of damage. Check the wiring connection to the flame rod. | Flame rod should be clean, properly positioned in the flame, and wiring should be secure. |
| Flame rod appears OK | Measure the flame rectification signal (DC microamps) at the control board during operation. | Typically, a reading of 2-5 uA DC indicates proper flame rectification. Consult the Takagi service manual for specific values. |
| Low or no flame rectification signal | Check the igniter for proper spark during startup. | A visible and consistent spark should be present at the igniter tip during the ignition sequence. |
Applicable Takagi Models
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Flame Rod Sensor | — | — | — |
| Gas Valve | — | — | — |
| Igniter | — | — | — |
Related Codes on Same Models
Other codes that affect the same Takagi model families as 121.
| Code | Description | Severity |
|---|---|---|
| 101 | Combustion Abnormality | Critical |
| 111 | Ignition 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 |
| 391 | Exhaust Temp Sensor | Warning |
| 510 | Gas Solenoid Valve | Critical |
| 610 | Fan Motor Fault | Warning |