Lennox GCWB95 40 — Return NTC Sensor Fault
40
Return NTC sensor fault. The return NTC sensor faulted; the manual directs calling an authorized service technician. The table classifies it as Error.
Applies to
This sensor fault code is displayed by the GCWB95 control (numeric error display with RESET) on GCWB95 models (GCWB95W-150, GCWB95W-205 wall hung combi and GCWB95F-150 floor mount combi), as documented in Lennox GCWB95 User's Information Manual, PN 240012894 Rev. A [04/15/2020], Section 3.6 error messages (models GCWB95W-150, GCWB95W-205 wall hung combi and GCWB95F-150 floor mount combi) (240012894 Rev. A). Codes can differ between controller generations, so always check the manual that matches your unit.
Controller: GCWB95 control (numeric error display with RESET). Source manual: 240012894 Rev. A (2020-04-15).
Why a code is not a diagnosis
A sensor code usually means the sensor, its wiring or its mounting is faulty; the physical condition the sensor measures (temperature, pressure, flow) may be perfectly normal.
Safe things to observe
- Note the code; there is no homeowner-accessible check for sensor circuits.
- If the appliance still runs, watch whether the code returns and report it when you call.
Do not
- Do not open the appliance to test or replace sensors yourself.
How professional service helps
- A technician measures the sensor and harness against the manual's values and replaces the faulty component.
- Combustion and safety devices are verified with an analyser and the manufacturer's procedure after any repair.
Where ROMA provides on-site service
On-site service is currently offered in Greater Vancouver, BC, including Vancouver, Burnaby, Richmond, Surrey, Coquitlam, North Vancouver, West Vancouver, New Westminster, Port Coquitlam, Port Moody, Delta, Langley, Maple Ridge, Abbotsford, Chilliwack, White Rock, Pitt Meadows, Tsawwassen, Anmore, Lions Bay, Belcarra, Bowen Island, Mission. Everything else on this site is reference information you can use anywhere in Canada.
Parts and service
Parts commonly involved
These parts appear in the documented text for this alert. A code reports a condition, not a failed part — a technician confirms the cause by testing before anything is replaced.
Thermistor / NTC sensor
A thermistor is the small temperature probe — technically a negative temperature coefficient, or NTC, resistor — that tells a boiler's control how…
What a technician checks →Boiler sensors (thermistors)
Boiler sensors are small temperature probes, usually thermistors, that tell the control how hot the water and flue gases are so it can fire safely…
What a technician checks →
Service for this alert
- Gas Boiler Repair
Licensed gas technicians from ROMA Heating & Cooling diagnose and repair residential and light-commercial gas boilers across…
Every boiler part explained — what it does, how it fails and what a technician checks.
Documented for Lennox Lennox GCWB95 (gas-fired condensing combination water boiler).
Frequently asked questions
Can I clear this code myself?
Clearing a code without fixing its cause usually just brings it back. Note the code, check the safe items on this page, then call a licensed technician. If your appliance manual describes a user reset for this exact code and it stays away, no call may be needed.
Boiler locked out? Call with the alert text ready.
The brand, model and this alert are passed along when you request service online, so the technician arrives prepared.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Sources and verification
- Lennox GCWB95 User's Information Manual, PN 240012894 Rev. A [04/15/2020], Section 3.6 error messages (models GCWB95W-150, GCWB95W-205 wall hung combi and GCWB95F-150 floor mount combi) · 240012894 Rev. A · accessed September 9, 2026
Facts checked September 9, 2026. Last updated September 9, 2026. Technical reviewer: not yet assigned.