Rinnai code 41: freeze protection sensor fault

Outdoor tankless heaters need to know the outside temperature so they can switch on freeze protection in time. Code 41 means the sensor that reports it has a problem.

The meaning and service steps below come from Rinnai's own diagnostic code guide for its tankless water heaters.

Why it matters

Without a working freeze protection sensor, the heater may not protect itself on a cold night. Rinnai notes that on some models an unplugged or damaged sensor stops the heater from firing, while on others the heater still runs and flashes code 41 alternating with the set temperature.

What you can check

• Look at whether someone wrapped the heater or its piping with insulation that now covers the sensor. Rinnai says the sensor must be open to outside air.
• Look for signs of animal damage to wiring under the unit.

What a technician does

The technician checks the wiring back to the control board and measures the sensor's resistance, which should drop as it warms. A failed sensor is replaced.

Before you call

Have the model and serial number from the rating label ready, along with the exact code and when it appears: at startup, during use, or only at high demand. Mention any recent work on the gas, water or electrical supply and when the heater was last descaled. This helps the technician bring the right parts and often saves a second visit.

See every code on our Rinnai error codes page.