Rinnai Rinnai RU199iN Sensei Super High Efficiency Plus Tankless Water Heater 34 Error Fix: Combustion Air Temp Sensor Failure

Error code 34 on the Rinnai Rinnai RU199iN Sensei Super High Efficiency Plus Tankless Water Heater means: Combustion Air Temp Sensor Failure. The combustion air temperature sensor has failed or is reading out of range. This sensor monitors the temperature of incoming combustion air to help the control board optimize the air-fuel ratio for efficient and clean combustion. On sealed combustion (direct-vent) installations, the combustion air comes from outside the building through a dedicated intake pipe. On natural draft or power-vent installations, combustion air is drawn from the surrounding space. Error 34 means the sensor is open, shorted, or reading an implausible temperature. On outdoor-installed models, extremely cold air temperatures can sometimes cause readings outside the sensor's expected range. Check the sensor wiring and connector. Test sensor resistance with a multimeter. Replace the sensor if readings are abnormal. This sensor failure typically causes the unit to stop operating for safety, as incorrect air temperature readings can lead to improper gas valve modulation.

The most successful fix is "Test and Replace the Combustion Air Temperature Sensor" (medium difficulty, ~50 minutes). Estimated DIY cost: $15-$25. 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 ("Test and Replace the Combustion Air Temperature Sensor") is rated medium difficulty and takes about 50 minutes. Tools needed: T20 Torx screwdriver, Digital multimeter, Needle-nose pliers.

DIY cost: $15-$25. Professional repair typically costs $140-$200. Many homeowners save money by trying the top-ranked community fix first.

The Rinnai Rinnai RU199iN Sensei Super High Efficiency Plus 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 34 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 RUR199iN Sensei Recirculating Tankless Water Heater, and 2 more. The fix may vary by model, so check each model's specific page for tailored repair steps.