Takagi Condensing Code 991
Abnormal combustion. The error that code 101 warns about. Takagi's diagnosis for the pair starts with blockage in the airway — vent obstructions, a backflow preventer installed backwards, the heat exchanger, the fan — then manifold pressure, contaminated intake air, weather, and altitude settings.
Call a proThe verdict
Call a licensed plumber
Abnormal combustion is the condition that produces carbon monoxide, and here the unit has stopped rather than warned. Do not reset it and carry on: get the vent system and the combustion side checked properly.
Typical cost: $150–$1,100
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the Condensing — not averaged across the brand.
Source: Takagi Condensing Models On-Demand Water Heater Troubleshooting Guide (p. 14), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
Before anything else: if you smell gas, stop here, leave, and call your gas utility.
- Stop using the unit and leave it off.
- Check that your carbon monoxide alarms work. If you have none, fit one before it runs again.
- Go outside and check the vent terminal for nests, trash, ice, or a frozen flap.
- Book a licensed professional.
Frequently asked questions
I had 101 for weeks and now I have 991.
That is the sequence working as designed. 101 is the warning, 991 is the stop, and the diagnosis is the same list — it just costs more to work through once the unit has quit.
Takagi mentions cleaning the burner. Can I do that?
No. It is inside the sealed combustion chamber of a gas appliance, and the procedure is written for a technician with the unit apart.
Code 991 on other Takagi series
The same code number can mean something different on another series — make sure you're reading the page for your exact unit: