Thermistor Failure
🔎What the code means
The heater has detected an abnormal or failed thermistor (temperature sensor) reading. The thermistor monitors temperature in the outlet or heat exchanger; when its resistance value is out of spec (open, shorted, out of range), the control board sets Code 311.
🔧 Error Code 311 – Causes & Troubleshooting Steps
Here are common root causes to investigate:
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The thermistor (sensor) itself is defective: internal failure, open circuit, shorted, or drifted outside of tolerance.
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Wiring issue: connector loose, broken wire, poor contact, corrosion, or damage causing incorrect signal to the board.
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Incorrect resistance reading due to sensor temperature being abnormal (for instance if the outlet is extremely hot or cold) but actual sensor is fine — though typically the sensor is faulty.
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For some installations: excessive scale/soot build-up, improper venting or heat exchanger overheating might cause sensor to be stressed or fail prematurely.
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A control board issue: less common, but if the board’s sensor read circuit is bad, it could display the code even if sensor wiring is OK.
🚫 Safety & Compliance Reminders
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Because this is a gas-fired condensing unit, unsafe conditions (e.g., overheating, flame/combustion problems) could exist. If you are not qualified, call a licensed technician.
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Some failures might lead to lower efficiency or unsafe operation of the heat exchanger. Don’t ignore persistent Code 311.
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Keep in mind other codes may accompany this issue; e.g., if wires are shorted you might see multiple codes.
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Preventive maintenance (cleaning filters, verifying water/flow/vent) helps avoid sensor stress.
Quick Links
🧰 STEP 1 — Power down the unit and verify safety (shut off gas, water, power) before any inspection.
🧰 STEP 2 — Locate the Thermistor
🧰 STEP 3 — Check Wiring & Connector
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Inspect for broken wires, loose connectors, corrosion, burnt contacts.
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Verify continuity (with unit powered off) from sensor connector back to control board.
🧰 STEP 4 — Measure Resistance
of the thermistor at the sensor connector (while disconnected) against the temperature table provided in the manual. For example, if the outlet is ~50°F (~10°C) the resistance might be around a specified value (check manual table).
- If the measured value is far out of spec (open circuit / very high resistance or very low resistance / shorted) → sensor likely bad.
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If the value is in spec but the code still persists → suspect wiring or board.
🧰 STEP 5 — Check Operating Conditions
- Ensure proper water flow, proper venting, no blockage, correct water temperature differential, no excessive scaling. These conditions prevent undue stress on sensors.
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Check for signs of overheating or unusual hot spots on the heat exchanger — can damage the thermistor.
🧰 STEP 6 — Replace Sensor if Defective
🧰 STEP 7 — Clear Fault Code
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(typically by cycling power) and monitor the unit under operation to verify code does not return.
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If after replacing sensor and checking wiring, the code persists → suspect the control board or sensor read circuit. At that point, board replacement may be required.