Rinnai code 33: Heat exchanger thermistor fault
Continuous flow hot water system (Australia), tankless water heater (United States)
The temperature sensor on the heat exchanger is giving no reading or an impossible one.
This needs a licensed gas technician. There is nothing safe to check yourself. Leave the covers on and book a professional.
What causes it
Australian models
-
Damaged sensor wiring, or a failed thermistor that needs replacing.
US models
-
Loose or damaged thermistor wiring, scale on the thermistor, or a failed thermistor.
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
73 models in 15 ranges
| Range | Models |
|---|---|
| Heavy Duty (commercial) | REU-AM3237WC (HD32e), REU-VCM2837FFC (HD28i), REU-VCM2837WC (HD28e), REU-NB3237WC-AK (Heavy Duty 32C), REU-NB3237FFC-AK, REU-NB3237WC-50-AK, REU-NB3237FFC-50-AK |
| 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), REU-NB3237W-AK (Infinity Enviro 32), REU-NB3237W-50-AK |
| 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) |
| SENSEI commercial | CU199i (REU-N3237FFC-US), CU160i (REU-N2530FFC-US), CU199e (REU-N3237WC-US), CU160e (REU-N2530WC-US) |
| SENSEI residential | RU199i (REU-N3237FF-US), RU180i (REU-N2934FF-US), RU160i (REU-N2530FF-US), RU130i (REU-N2024FF-US), RU199e (REU-N3237W-US), RU180e (REU-N2934W-US), RU160e (REU-N2530W-US), RU130e (REU-N2024W-US) |
| SENSEI residential with pump | RUR199i (REU-NP3237FF-US), RUR160i (REU-NP2530FF-US), RUR199e (REU-NP3237W-US), RUR160e (REU-NP2530W-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
- Infinity Enviro 32 / HDC32 operation and installation manual , page 12
- Troubleshooting tankless water heater diagnostic codes (100000652), non-SENSEI and SENSEI series , page 33
- N Series residential condensing installation and operation manual (100000467), RU199e/RU180e/RU160e/RU130e , page 60
Checked against the manufacturer's manual on 9 October 2026.
Other Rinnai tankless water heater codes
| Code | Meaning | What you can do |
|---|---|---|
| 18 | Gas valve adjustment limit | Call a pro |
| 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 |
| 34 | Temperature sensor fault (depends on model) | 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 |
See all temperature sensor fault codes on tankless water heaters across brands.