Takagi Error Code 121: Flame Loss (Flame Goes Out While Running)

What the code means

Takagi's troubleshooting guide for the 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) lists 121 as Flame Loss: the error appears if the flames disappear while the unit is still supposed to be in operation. On the 510 it shows as 121; on the 110 and 310 the remote shows 12 and the red LED on the computer board flashes three times. Takagi's service handbook for the 240, 340 and 540 condensing models groups it with 111 (ignition failure): 121 means the heater lost the flame signal during a heating cycle and couldn't re-establish a flame after three failed ignition attempts. The handbook says the cause can be inside the heater or external, such as no gas supply, and that the code resets on its own when water flow through the heater stops or the power is interrupted. Code 111 (or 11) is the related code for a heater that never lights at all, and it uses the same three flashes.

Likely causes

• A tripped hi-limit switch (often from scale buildup, or very hot incoming water such as from a solar preheat system)
• Not enough gas reaching the heater, or debris in the gas line
• A faulty pressure regulator at the gas meter
• On propane units, a cold regulator during colder parts of the day
• Gas supply pressure dropping out of spec while the heater runs (Takagi's handbook flags a drop of more than 1 to 1.5 inches of water column at full demand)
• A disconnected clear plastic tube between the proportional gas valve and the combustion chamber
• A dirty or damaged flame sensor (the flame comes on for only 1 to 2 seconds, then goes out)
• A blown overheat cutoff fuse (OHCF)
• A blocked vent (bird nests, animals, trash)
• A computer board, main gas valve or gas solenoid valve fault

Safe DIY checks

1. Turn off the hot water and turn it back on, or cycle the power. Takagi says the code clears when water flow stops or power is interrupted, so if it comes back the cause is still there.
2. Reset the hi-limit switch: turn the power to the heater off, push the white button in until you hear and feel it click, then turn the power back on and test. If the heater now runs, the reason it tripped still has to be found; Takagi's handbook says a repeat trip may mean the unit needs descaling, and to check that incoming water isn't preheated too hot.
3. Make sure the gas valve to the heater is fully open and, on propane, that the tank isn't low. If you smell gas, leave the house and call your gas company from outside.
4. Notice when it happens. If 121 shows up mostly when several hot taps run at once, or on cold mornings with propane, that points to gas supply or a cold regulator, which is worth telling your gas supplier or technician.
5. Check the outdoor vent termination for bird nests, animals, trash, snow or ice.
6. If the flame lights for only 1 to 2 seconds before going out, the flame sensor likely needs cleaning; Takagi covers this under cleaning the rod assembly.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 101 Takagi Tankless Water Heater 101
• 11 Takagi Tankless Water Heater 11
• 111 Takagi Tankless Water Heater 111
• 291 Takagi Tankless Water Heater 291
• 391 Takagi Tankless Water Heater 391

All Takagi codes →

Sources

1. Takagi Troubleshooting Guide 321923-000 for 110 (T-KJr2), 310 (T-K4) and 510 (T-D2): Table 2 error codes (121 on the 510, 12 on the 110/310 remote, three red LED flashes, Flame Loss; 111/11 Ignition Failure also three flashes); Error Code 121 / Three Flashes / 12 checklist (hi-limit reset, gas supply and debris, meter regulator, cold propane regulator, supply pressure in standby and running, clear plastic tube, flame sensor cleaning, OHCF, vent blockages, PC board or gas solenoid fault)
2. Takagi On-Demand Water Heater Service Handbook for 240, 340 & 540 Condensing Models (2000534342): Code 111 or 121, Ignition Failure / Flame Loss (lost flame signal and no flame after three ignition attempts, internal or external cause, automatic reset when flow stops or power is interrupted, 1 to 1.5 in. W.C. pressure drop, high limit switch and descaling, OHCF, air tube, spark, flame sensor, gas valve checks)