Takagi Water Heater Error Code 16: How to Fix It

Q: What does error code 16 mean on a Takagi tankless water heater?

Error code 16 on a Takagi tankless water heater means the flue thermistor has detected an abnormal temperature reading. This sensor monitors the temperature of the exhaust gases leaving the heat exchanger. The unit shuts down to protect itself when this reading falls outside the expected range, which can be caused by a blocked flue, insufficient ventilation, a faulty thermistor, or heavy scale buildup inside the heat exchanger.

Q: Can I reset error code 16 on my Takagi without fixing the underlying cause?

You can clear error code 16 temporarily by power cycling the unit, but if the root cause — such as a blocked vent, failing thermistor, or overheated heat exchanger — is not addressed, the error will return. Repeatedly resetting the unit without a fix risks heat exchanger damage and, in cases of vent blockage, a potential carbon monoxide hazard. Always investigate and resolve the cause before relying on a reset.

Q: How do I test the flue thermistor on a Takagi water heater?

Shut off power and gas to the unit, then remove the front access panel. Locate the flue thermistor near the exhaust outlet and disconnect its wiring connector. Set a multimeter to the resistance (ohms) setting and probe the two thermistor terminals. At room temperature, a functioning NTC thermistor should read a value in the kilohm range as specified in your Takagi service manual. A reading of zero ohms indicates a short circuit; an infinite reading indicates an open circuit — both mean the sensor has failed and needs replacement.

Q: How often should I descale my Takagi tankless water heater to prevent error code 16?

Takagi generally recommends descaling every 1–2 years, but homes with very hard water (above 120 mg/L or 7 grains per gallon) may need annual flushing. Scale buildup forces the heat exchanger to work harder, raising flue temperatures and increasing the chance of error 16. Installing an inline water softener or descaler ahead of the unit can significantly extend service intervals.

Q: Is error code 16 dangerous on a Takagi water heater?

Error code 16 itself is a protective measure — the unit shuts off to prevent overheating damage, so immediate physical danger from the heater is limited when it trips. However, if the root cause is a blocked or disconnected flue vent, there is a risk of carbon monoxide buildup in your home. If you smell combustion odors or suspect a venting problem, ventilate the area, do not attempt to restart the unit, and call a licensed technician immediately.

Abstract

Takagi error code 16 indicates that your tankless water heater's flue thermistor has detected an abnormal reading — either the sensor has failed, the connections are loose, or the flue is overheating due to a blockage or ventilation problem. This is a medium-severity fault that will typically shut down the unit to prevent overheating damage. Addressing it promptly protects the heat exchanger and ensures safe operation.