Rinnai code 34: Temperature sensor fault
Continuous flow hot water system (Australia), tankless water heater (United States)
This code means one of two things, depending on your model. Find your model family below, then read the section for it. The model number is on the rating label on the side of the unit.
Find your model family
| If you have | It means |
|---|---|
| HD32e, HD28i, HD28e, Infinity, Infinity B, Infinity Enviro 26, Infinity Enviro 16, Infinity internal, Commercial (KB), KB, KBD, KBP, KCM, Luxury (VC), Value (VC) | Combustion air temperature sensor fault |
| Heavy Duty 32C, REU-NB3237FFC-AK, REU-NB3237WC-50-AK, REU-NB3237FFC-50-AK, Infinity Enviro 32, REU-NB3237W-50-AK | Lime detecting thermistor fault |
Combustion air temperature sensor fault
On HD32e, HD28i, HD28e, Infinity, Infinity B, Infinity Enviro 26, Infinity Enviro 16, Infinity internal, Commercial (KB), KB, KBD, KBP, KCM, Luxury (VC), Value (VC).
The sensor that measures the temperature of incoming combustion air has failed or is reading out of range.
This needs a licensed gas technician. There is nothing safe to check yourself. Leave the covers on and book a professional.
Only some models show this code. Check the list below.
What causes it
Australian models
-
The combustion air temperature sensor or its wiring is faulty.
US models
-
Loose or damaged thermistor wiring, or a failed thermistor.
-
Wrong gas type or a botched gas conversion, incorrect dip switches, manifold pressures out of range, or venting and clearance problems.
How to read this code on your unit
Booking a licensed gas technician
Book a licensed gas technicianlicensed gas technician
- Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
- Note the code, how often it appears, and the model and serial numbers from the rating label.
- Book a licensed gas technician and give them those details.
Models that show this code
51 models in 12 ranges
| Range | Models |
|---|---|
| Heavy Duty (commercial) | REU-AM3237WC (HD32e), REU-VCM2837FFC (HD28i), REU-VCM2837WC (HD28e) |
| Infinity | REU-AM3237W (Infinity 32), REU-AM2626WDL (Infinity 26 Touch), REU-AM2626WD (Infinity 26), REU-AM2024WD (Infinity 20), REU-AM1620WD (Infinity 16), REU-AM1220WD (Infinity 12), REU-A2626WB (B26), REU-A2024WB (B20), REU-A1620WB (B16) |
| Infinity B | REU-BM2626W (Infinity B26), REU-BM2024W (Infinity B20), REU-BM1620W (Infinity B16), REU-BM1220W (Infinity B12) |
| Infinity Enviro | REU-E2626W (Infinity Enviro 26), REU-E1620W (Infinity Enviro 16) |
| Infinity internal | REU-VCM2837FF (Infinity 28i) |
| Commercial (KB) | C199i (REU-KBD3237FFUDC-US), C199e (REU-KB3237WDC-US) |
| KB | RU80i (REU-KB2530FFUD-US), RU80e (REU-KB2530WD-US), RU90e (REU-KBD2934WD-US), RU98i (REU-KB3237FFUD-US), RU98e (REU-KB3237WD-US) |
| KBD | RUC80i (REU-KBD2530FFUD-US), RUC90i (REU-KBD2934FFUD-US), RUC98i (REU-KBD3237FFUD-US) |
| KBP | RUR98i (REU-KBP3237FFUD-US), RUR98e (REU-KBP3237WD-US) |
| KCM | RUCS65i (REU-KCM2025FFU-US), RUCS75i (REU-KCM2528FFU-US), RUS65e (REU-KCM2025W-US), RUS75e (REU-KCM2528W-US) |
| Luxury (VC) | RL75i (REU-VC2528FFUD-US), RL75i(A) (REU-VC2528FFUD-US(A)), RL75e (REU-VC2528WD-US), RL75e(A) (REU-VC2528WD-US(A)), RL94i (REU-VC2837FFUD-US), RLX94i (REU-VC2737FFUD-US), RL94e (REU-VC2837WD-US) |
| Value (VC) | V53e (REU-VAM1620W-US), V53De (REU-AM1620WD-US), V65i (REU-VC2025FFU-US), V65e (REU-VC2025W-US), V75i (REU-VC2528FFU-US), V75e (REU-VC2528W-US), V94i (REU-VC2837FFU-US), V94Xi (REU-VC2737FFU-US), V94e (REU-VC2837W-US) |
Where this comes from
- Error codes for gas continuous flow water heaters (HW Gas CF ERR) , page 4
- Infinity gas continuous flow operation and installation manual , page 12
- Troubleshooting tankless water heater diagnostic codes (100000652), non-SENSEI and SENSEI series , page 34
Checked against the manufacturer's manual on 9 October 2026.
Lime detecting thermistor fault
On Heavy Duty 32C, REU-NB3237FFC-AK, REU-NB3237WC-50-AK, REU-NB3237FFC-50-AK, Infinity Enviro 32, REU-NB3237W-50-AK.
The thermistor the heater uses to detect lime (scale) build-up has reported a fault.
This needs a licensed gasfitter. There is nothing safe to check yourself. Leave the covers on and book a professional.
What causes it
-
A fault in the lime detecting thermistor or its wiring. Rinnai lists this as needing a service call.
How to read this code on your unit
Booking a licensed gasfitter
Book a licensed gasfitterlicensed gasfitter
- Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
- Note the code, how often it appears, and the model and serial numbers from the rating label.
- Book a licensed gasfitter and give them those details.
Models that show this code
6 models in 2 ranges
| Range | Models |
|---|---|
| Heavy Duty (commercial) | REU-NB3237WC-AK (Heavy Duty 32C), REU-NB3237FFC-AK, REU-NB3237WC-50-AK, REU-NB3237FFC-50-AK |
| Infinity Enviro | REU-NB3237W-AK (Infinity Enviro 32), REU-NB3237W-50-AK |
Where this comes from
Checked against the manufacturer's manual on 9 October 2026.
Other Rinnai tankless water heater codes
| Code | Meaning | What you can do |
|---|---|---|
| 19 | Electrical earth fault | Call a pro |
| 21 | Data transfer or dip switch error | Call a pro |
| 22 | Gas valve adjustment fault | Call a pro |
| 25 | Sensor fault or condensate blockage (depends on model) | Depends on your model |
| 32 | Outgoing water temperature sensor fault | Call a pro |
| 33 | Heat exchanger thermistor fault | Call a pro |
| 36 | Lower tank thermistor fault | Call a pro |
| 38 | Exhaust temperature thermistor fault | Call a pro |
| 41 | Frost protection (ambient) sensor fault | Call a pro |
| 51 | Inlet water thermistor fault | Call a pro |
| 52 | Gas modulating valve fault | Call a pro |
| 54 | High exhaust gas temperature | Safe checks first, then a pro |
See all temperature sensor fault codes on tankless water heaters across brands.