Rinnai Rinnai RUR199iN Sensei Recirculating Tankless Water Heater 31 Error Fix: Inlet Thermistor Failure

Error code 31 on the Rinnai Rinnai RUR199iN Sensei Recirculating Tankless Water Heater means: Inlet Thermistor Failure. The incoming (cold) water temperature sensor (thermistor) has failed or is reading out of range. The inlet thermistor measures the temperature of cold water entering the unit. The control board uses this reading along with the user's set temperature to calculate how much heat must be added. If the sensor fails (open circuit, short circuit, or reading outside the expected range of 32-120°F), the unit cannot properly modulate the gas valve and will shut down with error 31. The inlet thermistor is typically a small probe inserted into the cold water pipe inside the unit, connected to the control board by a two-wire connector. Diagnosis: disconnect the thermistor leads from the control board and measure resistance with a multimeter. At 77°F (25°C), resistance should be approximately 10,000 ohms (10K). If the reading is infinite (open), zero (short), or vastly different from the expected value, replace the thermistor. Rinnai thermistors are inexpensive parts ($15-30) and straightforward to replace.

The most successful fix is "Replace the Inlet Thermistor Sensor" (medium difficulty, ~30 minutes). Estimated DIY cost: $15-$30. HomeMD has 2 community-ranked fixes for this error.

This error is rated warning severity. It is generally not urgent and can often be resolved with a simple DIY fix.

Yes — the top-ranked community fix ("Replace the Inlet Thermistor Sensor") is rated medium difficulty and takes about 30 minutes. Tools needed: Phillips screwdriver, Needle-nose pliers, Multimeter.

DIY cost: $15-$30. Professional repair typically costs $130-$190. Many homeowners save money by trying the top-ranked community fix first.

The Rinnai Rinnai RUR199iN Sensei Recirculating Tankless Water Heater has 8 other known error codes, including: 02 (Ignition Failure), 03 (Bath Fill Function Abnormality), 05 (Blocked Exhaust / Air Supply), 10 (Blocked Exhaust — Air Pressure Switch), 11 (No Ignition), and 3 more.

Error code 31 also appears on 8 other Rinnai models: Rinnai RE180iN RE Series Non-Condensing Tankless Water Heater, Rinnai RL75iN RL Series Tankless Water Heater, Rinnai RL94iN RL Series Tankless Water Heater, Rinnai RSC160iN Condensing Combi Boiler, Rinnai RU130iN Sensei Super High Efficiency Plus Tankless Water Heater, Rinnai RU199iN Sensei Super High Efficiency Plus Tankless Water Heater, and 2 more. The fix may vary by model, so check each model's specific page for tailored repair steps.