Rinnai Water Heater Error Code 15: How to Fix It

Q: What does error code 15 mean on a Rinnai water heater?

Error code 15 on a Rinnai tankless water heater or boiler indicates a thermistor fault. The thermistor is a temperature sensor that monitors water or heat exchanger temperature. When the control board receives a reading that is out of the expected range — or no reading at all — it triggers code 15 and shuts down heating to prevent unsafe operation.

Q: Can I still use my Rinnai water heater when error code 15 is showing?

No. When error code 15 is active, the Rinnai unit will typically lock out and stop heating water as a safety measure. You should resolve the fault before continuing to use the unit, as operating with a faulty temperature sensor could result in overheating or unpredictable water temperatures.

Q: How do I reset error code 15 on my Rinnai?

Press and hold the On/Off button on the controller for a few seconds to attempt a reset, or turn off power at the circuit breaker for 30 seconds and restore it. However, if the underlying thermistor fault is not fixed, the code will return as soon as the unit tries to operate. A reset alone is not a permanent solution.

Q: How much does it cost to fix a Rinnai error code 15?

If you replace the thermistor yourself, the part typically costs between $20 and $60 depending on your model. If you hire a licensed technician, expect to pay $100–$250 or more in labor on top of the part cost. If the control board is also damaged, total repair costs can be higher.

Q: Could error code 15 be caused by something other than a bad thermistor?

Yes. A loose or corroded wire connector at the thermistor plug is a very common cause and is often overlooked. A damaged wire harness, a shorted circuit, or in rare cases a failing control board that misreads the sensor signal can also trigger code 15. Always check the wiring connections first before replacing the thermistor itself.

Abstract

Rinnai error code 15 indicates a thermistor fault on your tankless water heater or boiler — meaning the temperature sensor is sending an abnormal or out-of-range reading to the control board. This can be caused by a faulty thermistor, loose wiring connections, or a damaged wire harness. Left unaddressed, this fault will prevent your unit from heating water safely and reliably.