HVACopediaRinnai · Tankless water heater
38 Exhaust gas temperature sensor (CO sensor on RH180)
What the control is telling you
The exhaust gas temperature sensor reports a fault or abnormal flue conditions; on RH180 models this code relates to the CO sensor and venting/combustion air problems.
Applies to: SENSEI Series; SENSEI RX/CX Series; Non-SENSEI Series; RH180 (CO sensor steps)
Likely causes
- Loose or damaged sensor wiring, or sensor out of spec
- Wrong gas type or improper gas conversion
- Incorrect parameters/DIP switches or manifold gas pressures
- Debris in burner, manifold or heat exchanger fins
- Venting clearances, lengths or materials not per spec
- Insufficient combustion air (RH180)
- Restricted fan wheel, housing or air duct (RH180)
- Failed CO sensor or PC board (RH180)
Test and repair steps
- Inspect sensor wiring at sensor and PC board. Remove sensor and test at 20K ohms: 11.4–14K 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; should fall when warmed.
- If readings are good, reinstall the sensor in the exhaust port and run water to fire the unit; continue if code 38 returns.
- Confirm gas type matches the rating plate, any gas conversion was done per the Gas Conversion manual, clearances are met, and parameters/DIP switches suit the site.
- Warning: Only technicians qualified on gas systems should continue. Verify forced low/high fire manifold pressures with a manometer per the Technical Data Sheet.
- Warning: Shut off gas and water first. Inspect burner manifold, burners and heat exchanger copper fins for debris; confirm venting clearances and lengths. If code 38 remains, replace the sensor.
- RH180 Confirm 4 in. B-Vent was used and the full vent system is properly installed, seated and sealed with correct clearances.
- RH180 Check combustion air for all gas appliances in the space; if the unit runs with a door open, recalculate combustion air requirements.
- RH180 Set altitude DIP switches 2 and 3: 0–2,000 ft both OFF; 2,001–5,400 ft SW2 OFF, SW3 ON.
- RH180 (WARNING, qualified gas techs only): Inlet gas NG 4–10.5 in. w.c., LP 8–13.5 in. w.c. with all appliances on high fire; forced low NG 1.10/LP 1.23, forced high NG 2.8/LP 3.0 in. w.c.
- RH180 (CAUTION, power off) Remove the fan and inspect wheel, housing and air duct for restrictions; reinstall, restore power and test.
- RH180 (WARNING, power/gas/water off) Check manifold orifices, burner chamber and HX fins for debris; blow out with 120 psi air or replace the heat exchanger.
- Caution: Wear safety glasses and a mask when blowing out the burner.
- RH180 Review the RH180 steps under Codes 05 and 10. Check CO sensor at C1, Red–Black pins 1–5: 1.9–2.1 VDC normal, 2.5–3 VDC during combustion.
- RH180 Before replacing the CO sensor, ventilate the room, cycle power, wait 30 minutes and retry. If unresolved, replace the CO sensor and PC board.
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
- 32 Outlet water temperature thermistor fault
- 33 Heat exchanger thermistor fault
- 34 Combustion air thermistor (Non-SENSEI) / Lime detecting thermistor (RX/CX)
- 35 Tank temperature sensor fault
- 41 Freeze protection / outdoor air temperature sensor fault
- 51 Inlet water temperature thermistor fault
- 52 Gas valve / modulating solenoid valve (POV) signal fault
- 54 High exhaust gas temperature
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Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.42–43. Summarized in our own words; check the original manual for your model.
Homeowner looking at this code? See what it means for you and when to call a technician.
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.