Takagi Error Code 99: Abnormal Combustion on the 110 (T-KJr2) and 310 (T-K4)
What the code means
Takagi's troubleshooting guide for the 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) lists code 99 as Abnormal Combustion. On the 110 and 310 the remote controller shows 99 and the red LED on the computer board flashes five times; the 510 shows the same problem as code 991. Code 10 (101 on the 510) is the warning that comes before it: it also flashes five times, and Takagi says a 10/101 reduces the amount of gas going into the heater. If the cause isn't fixed, it becomes 99/991. Takagi's steps for 99 are to work through the 10/101 checklist, contact its Technical Service Department if nothing works (the manifold gas pressure may need adjustment), and reset the computer board once you're sure the problem is solved.
Likely causes
- A blocked vent: birds' nests, trash or other obstructions in the vent pipe
- A backflow preventer installed backwards
- Dirt or blockage in the heat exchanger or fan
- 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 (FM+) DIP switch setting
- Other DIP switches set wrong for the installation, such as gas type
- Manifold gas pressure that's out of adjustment
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.
- Go outside and check the vent termination. Clear any nests, leaves, trash, snow or ice, and make sure the termination flap isn't frozen shut. Takagi lists high winds as a cause too, so note whether the code shows up on windy days.
- Look at where the heater sits. If it's near a dryer or washer, in a kitchen, or somewhere dusty, lint or grease has probably gotten into the burner; Takagi's guide covers cleaning the burner and rod assembly, 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 is new, ask the installer whether the high-altitude (FM+) and gas type DIP switches were set for your location. Takagi says high altitude can cause a lack of air.
- After the cause is fixed, reset the computer board: make sure no water is running through the heater, turn off the power to the remote controller if you have one, turn the heater's power off and back on, hold the INC and DEC buttons on the computer board together for several seconds until the red LED lights up solid, then turn the heater off and on again with its power switch. This means opening the front cover, so leave it to a technician if you'd rather not.
- If 99 comes back after a reset, stop resetting it and get the heater serviced. Bad combustion can mean a blocked vent pushing exhaust indoors, so make sure you have a working carbon monoxide alarm.
🛠 When to call a pro
- Have a technician check for a backflow preventer installed backwards and for blockages in the heat exchanger and fan.
- Have the burner and rod assembly cleaned if the intake air is contaminated with dust, lint or grease.
- Have a technician confirm the DIP switch settings for gas type, model and altitude against Takagi's tables for your model.
- Manifold gas pressure should only be checked and adjusted by a qualified technician; Takagi says to contact its Technical Service Department if nothing else has worked, since the manifold pressure may need adjustment.
Parts that commonly fix it
Always match the part number to your exact model.
Carbon monoxide alarm
Strongly recommended: 99 means the heater detected bad combustion, and a blocked vent 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
Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.