Takagi Condensing Code 101
Abnormal combustion (warning). The warning that precedes error 991. Takagi's own checks for it start with blockage in the airway — vent obstructions such as birds' nests or trash, a backflow preventer installed backwards, the heat exchanger, the fan — then manifold pressure, then contaminated intake air from dust, lint, or kitchen grease, then weather and altitude settings.
Call a proThe verdict
Call a licensed plumber
Abnormal combustion is the condition that produces carbon monoxide, and this is the polite version of it before the unit stops. Check the vent terminal outside, then get it inspected — most of Takagi's list needs the unit open and a manometer.
Typical cost: $150–$900
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.
- Go outside and check the vent terminal for nests, trash, ice, or a flap that has frozen shut — Takagi's first cause.
- Check that your carbon monoxide alarms work.
- Note whether the unit is near a laundry, a kitchen, or anywhere else with dust, lint, or grease in the air — Takagi lists contaminated intake air as a cause.
- Book a licensed professional.
Frequently asked questions
Is 101 the same as 991?
Same subject, different stage. Takagi's table calls 101 the warning for the 991 error code, and gives 101 the full abnormal-combustion diagnosis. Dealing with it at 101 is the cheap version.
Altitude is mentioned. Why?
Above about 3,000 feet Takagi expects DIP switches 5 and 6 on the left bank to be changed. Thinner air needs a different air-fuel balance, and a unit set for sea level burns badly at altitude.
Code 101 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: