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
Other Takagi series report code 991 under the same fault name, but each manual is transcribed separately and they do not read alike:
Don't confuse this with
One other Takagi Condensing code points at combustion itself too. They are the same symptom reported from a different place in the unit, so which one the display shows is what tells you where to look: