34 Combustion air thermistor (Non-SENSEI) / Lime detecting thermistor (RX/CX)

What the control is telling you

On Non-SENSEI units, the combustion air temperature thermistor reports a fault or abnormal combustion conditions; on SENSEI RX/CX units, the lime detecting thermistor on the hot water pipe is faulty.

Applies to: Non-SENSEI Series; SENSEI RX/CX Series

Likely causes

• Loose, broken or damaged thermistor harness
• Thermistor resistance out of spec or failed thermistor
• Wrong gas type or improper gas conversion (Non-SENSEI)
• Incorrect DIP switch settings or manifold gas pressures (Non-SENSEI)
• Debris in burner, manifold or heat exchanger fins (Non-SENSEI)
• Inadequate clearances or venting out of spec (Non-SENSEI)
• Foreign material on lime thermistor (RX/CX)

Test and repair steps

1. Non-SENSEI Inspect the thermistor harness to the PC board, remove the thermistor from the fan motor housing, and test it with a meter set for 20K ohms (resistance should fall when warmed).
2. Non-SENSEI Typical values 11.4–14K ohms 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.
3. Non-SENSEI Confirm the unit matches the gas type on the rating plate; if it was ever converted, confirm the model-specific procedure in the Gas Conversion manual was followed.
4. Non-SENSEI Confirm clearances around the unit and vent terminals, and that DIP switches match the Gas Conversion or Installation manual.
5. Warning: Only technicians qualified on gas systems should continue.
6. Non-SENSEI With a manometer, verify forced low and high fire manifold pressures per the Technical Data Sheet inside the front cover.
7. Warning: Shut off gas and water supply to the unit before continuing.
8. Non-SENSEI Inspect burner manifold, burner assembly and heat exchanger copper fins for debris or blockage, and confirm venting clearances and lengths meet spec.
9. Non-SENSEI If code 34 remains, replace the thermistor.
10. SENSEI RX/CX Inspect the lime thermistor harness to the PC board, then remove the thermistor from the hot water pipe.
11. SENSEI RX/CX Test with a meter set for 100K ohms. Typical: 70.6–85.0K at 59°F, 36.5–42.8K at 86°F, 20.0–22.8K at 113°F, 11.5–12.8K at 140°F, 2.7–3.0K at 221°F.
12. SENSEI RX/CX Clean any foreign material off the thermistor, reinstall it on the hot water pipe, and replace it if code 34 continues.

Settings for these units

RX supply connection sizes · SENSEI RX/CX

Vent connection: 2 in. nominal PVC/CPVC/PP or 3 in./5 in. concentric.

Full specs (input, flow, weight) are only on each model's spec sheet, not in this manual.

RX water quality limits · SENSEI RX/CX

Water damage beyond these limits is not covered by warranty.

• RX water treatment and flushing by hardness · SENSEI RX/CX
• RX clearances to combustibles · SENSEI RX/CX
• RX temperature settings available · SENSEI RX/CX
• RUCS/RUS unit clearances · Non-SENSEI
• RUCS/RUS water quality limits · Non-SENSEI

Models that show this code

69 models in Non-SENSEI, RH180 hybrid, SENSEI RX/CX

Other codes on the same units

• 30 Flammable vapor sensor activated or faulty
• 31 Burner thermocouple out of range
• 32 Outlet water temperature thermistor fault
• 33 Heat exchanger thermistor fault
• 35 Tank temperature sensor fault
• 38 Exhaust gas temperature sensor (CO sensor on RH180)
• 41 Freeze protection / outdoor air temperature sensor fault
• 51 Inlet water temperature thermistor fault