**A failed thermocouple is a likely cause, but this symptom alone doesn’t prove it needs replacing.**

Holding the knob down manually opens the pilot gas valve. Once you release it, the heated thermocouple must generate enough electrical current to keep the safety valve open. If that signal is insufficient—or the valve cannot hold—the gas shuts off immediately. Two minutes would ordinarily be enough warm-up time; follow your furnace’s lighting instructions. [Resideo gas-control manual](https://customer.resideo.com/resources/Techlit/TechLitDocuments/60-0000s/60-2019.pdf)

The main possibilities are:

- **A worn or failed thermocouple.**
- **A weak or misdirected pilot flame** that isn’t adequately heating the thermocouple tip, potentially from a dirty pilot opening.
- **A loose thermocouple connection** at the gas valve.
- **A faulty safety mechanism inside the gas valve.** [Carrier pilot-light guidance](https://www.carrier.com/us/en/residential/hvac-resources/furnaces/furnace-pilot-light/), [Resideo troubleshooting](https://customer.resideo.com/resources/Techlit/TechLitDocuments/60-0000s/60-2019.pdf)

You can visually check whether the pilot burns steadily blue and surrounds the thermocouple tip. Have an HVAC technician check the flame, connection, and thermocouple output before replacing parts. **Replacement is warranted if its output is inadequate despite proper flame heating.** Meanwhile, leave the furnace control off; never wedge the knob down or bypass the safety device.

If you smell gas, stop trying to light it, leave the building without operating electrical switches, and call your gas supplier’s emergency number from outside. [Gas-emergency guidance](https://www.nationalgridus.com/Report-Gas-Emergency)