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Rinnai · Tankless water heater

41 Freeze protection / outdoor air temperature sensor fault

Medium severity Non-SENSEIRH180 hybridSENSEI RX/CXSENSEI

What the control is telling you

The freeze protection (outside air) thermistor is open, shorted or disconnected. KB units will not fire; VC units and outdoor units keep running while flashing 41 alternately with the set point.

Applies to: SENSEI and Non-SENSEI Series (external/outdoor models); all SENSEI RX/CX Series models

Likely causes

  • Loose, broken or damaged sensor harness
  • Sensor unplugged, cut or shorted
  • Sensor covered by insulation instead of exposed to outside air
  • Sensor resistance out of spec
  • Temperature controller wired to the wrong terminals

Test and repair steps

  1. Inspect the sensor harness from sensor to PC board for loose, broken or damaged connections.
  2. Make sure the sensor is exposed to outside air and not buried in insulation.
  3. With a meter set for 20K ohms, measure across the sensor plug; resistance should drop when warmed and rise when cooled.
  4. Typical values 11.4–14K ohms at 59°F, 6.4–7.8K at 86°F, 3.6–4.5K at 113°F, 2.2–2.7K at 140°F, 0.6–0.8K at 221°F.
  5. Note An open sensor on outdoor units (all RX/CX units) lets the heater keep running while flashing 41; the code clears once the issue is fixed.
  6. Confirm the temperature controller is wired to the connection point shown in the Installation and Operation manual; if it is connected anywhere else, disconnect it immediately and rewire correctly.

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Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.44. Summarized in our own words; check the original manual for your model.

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