Takagi tankless water heaters display 3-digit numeric error codes on the controller. The first digit indicates the system area (1xx = ignition/flame, 3xx = sensor, 6xx = combustion air, 9xx = exhaust). Codes are consistent across T-H3, T-D2, T-K, T-KJr, and T-T lines.
The top codes you'll actually see
| Code | Meaning | Most common cause |
|---|---|---|
| 111 | Ignition failure | Gas supply / igniter / flame rod |
| 112 | Flame failure during operation | Dirty flame rod / vent |
| 121 | Overheat / heat exchanger | Scaled heat exchanger |
| 311 | Outlet thermistor | Sensor or wiring |
| 651 | Combustion fan | Fan motor / RPM |
| 991 | Exhaust restriction | Vent blockage / icing |
Full numeric reference
| Code | Description |
|---|---|
| 101 | Air supply abnormality |
| 111 | Ignition failure / no flame |
| 112 | Flame failure during operation |
| 121 | Heat exchanger overheat |
| 211 | Solenoid valve |
| 311 | Outlet thermistor |
| 312 | Inlet thermistor |
| 321 | Combustion air thermistor |
| 322 | Bypass servo |
| 331 | Heat exchanger thermistor |
| 441 | Flow control valve |
| 551 | Cascade communication (T-T) |
| 611 | Modulating gas valve |
| 651 | Combustion fan motor |
| 661 | Combustion fan RPM sensor |
| 701 | Control board (PCB) |
| 711 | Solenoid valve circuit |
| 721 | Flame sensing circuit |
| 761 | Remote controller |
| 901 | Combustion abnormality |
| 991 | Exhaust blockage / vent restriction |
| LC | Low water flow |
| HC | High water flow |
How to reset
Three levels:
- Level 1: press the On/Off button on the controller for 3 seconds
- Level 2: turn off breaker, wait 30 seconds, restore
- Level 3: service menu — requires installer password
If the code returns within 60 seconds of reset, the underlying fault is active. Work through the specific code's diagnostic walkthrough.
Bottom line
Each code has a specific cause and diagnostic sequence. Click the linked codes for the walkthrough. For cases without a code, see general troubleshooting.