HVACopediaRinnai · Tankless water heater
41 Freeze protection / outdoor air temperature sensor fault
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
- Inspect the sensor harness from sensor to PC board for loose, broken or damaged connections.
- Make sure the sensor is exposed to outside air and not buried in insulation.
- With a meter set for 20K ohms, measure across the sensor plug; resistance should drop when warmed and rise when cooled.
- 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.
- 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.
- 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.
Settings for these units
RSC supply connection sizes · SENSEI
| Connection | Size |
|---|---|
| Gas | 3/4 in. NPT |
| Cold inlet | 3/4 in. NPT |
| Hot outlet | 3/4 in. NPT |
| Condensate drain | 1/2 in. NPT |
| Pressure relief valve | M33 thread |
The PRV and its adapter come in the box; PRV must be rated 150 psi and at least the heater Btu/h.
Condensate drain line must stay at least 1/2 in. over its whole length.
RSC water quality limits · SENSEI
| Contaminant | Maximum level |
|---|---|
| Total hardness | Up to 200 mg/L |
| Aluminum | Up to 0.2 mg/L |
| Chlorides | Up to 250 mg/L |
| Copper | Up to 1.0 mg/L |
| Dissolved CO2 | Up to 15.0 mg/L |
| Iron | Up to 0.3 mg/L |
| Manganese | Up to 0.05 mg/L |
| pH | 6.5 to 8.5 |
| TDS | Up to 500 mg/L |
| Zinc | Up to 5 mg/L |
Damage from water above these limits is not covered by warranty.
Hard water should be treated; use the Service Soon (SS) reminder (parameter 03) for flushing intervals.
- RSC clearances to combustibles · SENSEI
- RSC temperature settings available · SENSEI
- RX supply connection sizes · SENSEI RX/CX
- RX water quality limits · SENSEI RX/CX
- RX water treatment and flushing by hardness · SENSEI RX/CX
- RX clearances to combustibles · SENSEI RX/CX
Models that show this code
89 models in Non-SENSEI, RH180 hybrid, SENSEI RX/CX, SENSEI
Other codes on the same units
- 33 Heat exchanger thermistor fault
- 34 Combustion air thermistor (Non-SENSEI) / Lime detecting thermistor (RX/CX)
- 35 Tank temperature sensor fault
- 38 Exhaust gas temperature sensor (CO sensor on RH180)
- 51 Inlet water temperature thermistor fault
- 52 Gas valve / modulating solenoid valve (POV) signal fault
- 54 High exhaust gas temperature
- 57 Burner overheat bi-metal switch open
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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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For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.