Takagi Error Code 101: Abnormal Combustion Warning (Before a 991 Shutdown)
What the code means
Takagi's condensing service handbook describes code 101 as a warning that the air-fuel ratio for combustion is gas rich. The heater continues to operate, but shutdown is imminent: the next step is code 991, which forces the heater off. On the 510 (T-D2) the code shows as 101; on the 110 (T-KJr2) and 310 (T-K4) the remote shows 10 and the red LED on the computer board flashes five times (the same flash count as 991/99). Takagi's troubleshooting guide also notes that a 101 reduces the amount of gas going into the heater, so lukewarm water can be a symptom. Once the cause is found and fixed, the computer board has to be reset to clear the code.
Likely causes
- A blocked vent or intake: birds' nests, trash, or construction debris in the vent pipe
- A backflow preventer installed backwards
- Dirt or a partial blockage in the heat exchanger or fan, so the fan can't push enough air
- Intake air contaminated with dust, lint (a laundry machine nearby), grease (a kitchen) or other contaminants, fouling the burner
- Weather at the vent, such as high winds or a frozen vent termination flap
- An installation around or above 3,000 feet without the high-altitude DIP switch setting
- DIP switches set wrong for the gas type or installation
- Undersized gas lines or manifold gas pressure that's out of adjustment
- Vent terminations too close to each other or to another appliance's exhaust, so flue gas gets pulled back into the intake
Safe DIY checks
Unplug the appliance (or switch off its breaker) and shut off the water supply valves before removing panels or disconnecting hoses. Keep towels handy for residual water.
- Treat 101 as an early warning: the heater still works, but Takagi says a 991 shutdown follows if nothing is done, so deal with it now rather than waiting.
- Go outside and check the vent and intake terminations. Clear nests, leaves, trash, snow or ice, and make sure the termination flap isn't frozen shut. If the code shows up on windy days, note that for your installer.
- Check that no other appliance's exhaust (a dryer vent, another water heater or furnace) sits close enough to the heater's intake to be pulled in.
- Look at where the heater is installed. If it's near a washer or dryer, in a kitchen, or somewhere dusty (including after drywall work), lint, grease or dust has probably gotten into the fan and burner; Takagi calls for cleaning the burner and fan, which is a job for a technician if you're not comfortable opening the heater.
- If you live around or above 3,000 feet, or the heater was just installed, ask the installer whether the altitude and gas type DIP switches were set for your location.
- After the cause is fixed, the code has to be cleared by resetting the computer board inside the heater (on condensing models: standby light off, hold INCREASE and DECREASE on the board for five seconds, then power the heater off and back on). This means opening the front cover, so leave it to a technician if you'd rather not.
- Make sure you have a working carbon monoxide alarm. A gas-rich flame and blocked venting can push exhaust indoors.
🛠 When to call a pro
- Have a technician check the vent length and layout against the installation manual (Takagi counts each 90-degree elbow as 5 feet of straight pipe) and check for a backwards backflow preventer.
- Have the fan, burner and heat exchanger air passages inspected and cleaned if there's dust, lint or debris inside the cabinet.
- Have the inlet gas pressure checked under maximum demand; Takagi says a drop of more than about 1 inch of water column points to undersized gas lines or a supply problem.
- Manifold gas pressure should only be checked and adjusted by a qualified technician, and Takagi's handbook says to do it only with approval from its Technical Support.
Parts that commonly fix it
Always match the part number to your exact model.
Carbon monoxide alarm
Strongly recommended: 101 means the heater is burning gas rich, and blocked venting can push exhaust indoors.
Search eBay →Gas leak detector
Useful when checking gas lines; leave the house and call the gas company if you smell gas.
Search eBay →Sources
- Takagi On-Demand Water Heater Service Handbook for 240, 340 & 540 Condensing Models (2000534342): Code 101, Abnormal Combustion, warning that the air-fuel ratio is gas rich, heater continues to operate but shutdown is imminent, next step is 991 which forces shutdown, undersized gas lines can cause 101 or 991; error table lists five flashes; shared 101/991 steps (gas type and DIP switches, vent length with 90-degree elbow = 5 ft, elevation, gas pressure drop over 1 in. W.C., vent/intake obstructions, termination clearances and other appliances' exhaust, contaminated air in cabinet and fan making burners gas rich, manifold pressure only with Technical Support approval, 101/991 reset procedure)
- Takagi Troubleshooting Guide 321923-000 for 110 (T-KJr2), 310 (T-K4) and 510 (T-D2): Error Code 101 (510) / five flashes / 10 (110, 310), Abnormal Combustion; checklist: airway blockage (vent obstruction, backflow preventer backwards, heat exchanger, fan), manifold pressure, contaminated intake air and burner cleaning, weather (high winds, frozen vent termination flap), altitude around or over 3,000 ft and FM+ DIP switches, DIP switch settings, reset the computer board; Table 2 lists 101/10 as Warning for 991; a 101 reduces the gas input into the heater
Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.