Takagi Error Code 991: Abnormal Combustion (Heater Shut Down)

What the code means

Takagi's service handbook for its condensing models (240, 340 and 540) says code 991 indicates imperfect combustion, which can be caused by insufficient intake air or exhaust obstructions. It's the second stage of code 101: 101 is a warning that the air-fuel ratio is gas rich and the heater keeps running, but if the problem isn't fixed it moves on to 991, which forces the water heater to shut down. Takagi notes undersized gas lines can cause either code. On the 510 (T-D2) and 910 (T-M50) the display shows 991; the 110 (T-KJr2) and 310 (T-K4) show five flashes on the board LED or code 99 on the remote. The condensing models also show five flashes on the board's green LED. Don't confuse it with 391 (air-fuel ratio rod failure) or 111 (ignition failure).

Likely causes

• A blocked vent or air intake, such as a bird's nest, trash, or a frozen vent termination flap
• Venting that's too long or the wrong size for the DIP switch settings (Takagi counts each 90-degree elbow as 5 feet of straight pipe)
• Dust, lint (a dryer nearby), grease (a kitchen) or other contaminants in the intake air, clogging the fan, burner or heat exchanger fins
• A gas line, gas meter or regulator that's too small for the heater
• Exhaust from the heater or another appliance being pulled back into the air intake
• High winds, or an installation above 2,000 feet (condensing models) or around 3,000 feet (older models) without the high-altitude DIP switch setting
• Manifold gas pressure that's out of adjustment

Safe DIY checks

1. Go outside and look at the vent and air intake terminations. Clear any nests, leaves, trash, snow or ice, and make sure the termination flap isn't frozen.
2. Check whether another appliance's exhaust (a furnace, dryer or another water heater) vents close enough to be pulled into the Takagi's intake, and whether the terminations meet the clearances in the installation manual.
3. Look at where the heater sits. If it's near a dryer, a kitchen, a workshop or anywhere dusty, lint or grease in the cabinet or fan is a likely cause, and Takagi says the fan and burner should be cleaned.
4. If the heater was installed recently or a gas appliance was added, ask whether the gas line was sized for it. Takagi says a pressure drop of more than 1 inch W.C. between standby and full firing points to an undersized line, meter or regulator.
5. After the cause is fixed, clear the code. On condensing models, with the heater powered on, press the on/off button on the remote so the amber standby light is off, hold the INCREASE and DECREASE buttons on the computer board together for five seconds until the green LED goes off, then turn the power off at the switch above the board and back on after 5 seconds. Older models have their own board reset procedure in the troubleshooting guide. If you're not comfortable opening the front cover, leave this to a technician.

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
• 121 Takagi Tankless Water Heater 121
• 291 Takagi Tankless Water Heater 291

All Takagi codes →

Sources

1. Takagi On-Demand Water Heater Service Handbook for 240, 340 & 540 Condensing Models (2000534342): Code 991, Abnormal Combustion, imperfect combustion caused by insufficient intake air or exhaust obstructions; Code 101 warning that the air-fuel ratio is gas rich, next step is 991 which forces shutdown, undersized gas lines can cause 101 or 991; error table lists five flashes; steps: gas type and DIP switch, venting, vent length (90-degree elbow = 5 ft), elevation (default 2,000 ft), gas pressure drop over 1 in. W.C., vent/intake obstructions, termination clearances and other appliances' exhaust, contaminated air in cabinet and fan, manifold pressure with Technical Support approval, 101/991 reset procedure
2. Takagi Troubleshooting Guide 321923-000 for 110 (T-KJr2), 310 (T-K4) and 510 (T-D2): Error Code 991 (510) or five flashes / 99 (110, 310), Abnormal Combustion, refer to error code 101 (airway blockage: vent obstruction, backflow preventer backwards, heat exchanger, fan; manifold pressure; contaminated intake air; weather such as high winds or frozen vent flap; altitude around or over 3,000 ft; DIP switches), contact Technical Service, manifold pressure may need adjustment, reset the computer
3. Takagi Troubleshooting Guide 321925-000 for 910 (T-M50): Error Code 991, Abnormal Combustion, same steps as 101