Takagi Non-Condensing Code 101
Abnormal combustion (warning). The warning that precedes error 991 — the unit is telling you combustion is not right while still running. Shown as 10 on a T-K4 or T-KJr2 remote, or five flashes of the red LED.
Also displayed as: 10
Call a proThe verdict
Call a licensed plumber
Abnormal combustion is the condition that produces carbon monoxide, and this is the polite version before the unit stops. Check the vent terminal outside, then get it inspected — the rest of the diagnosis 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 Non-Condensing — not averaged across the brand.
Source: Takagi Non-Condensing Models On-Demand Water Heater Troubleshooting Guide (p. 12), 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 frozen flap.
- Check that your carbon monoxide alarms work.
- Note whether the unit is near a laundry, kitchen, or anywhere with dust, lint, or grease in the air.
- 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. Dealing with it at 101 is the cheap version.
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: