What Effect thermo coupling has if broken?

The thermocouple generates a small amount of electricity when it is heated which sticks open an electromagnetic valve,
If the thermocouple fails, you'll be able to light the pilot, but it will shut down when you release the button.
They're easy enough to replace, and they're under $20.
Remember, this is an electrical connection, not gas. You don't have to sock it down or use sealer.