To replace a gas fireplace thermocouple, take measurements of the removed part, or carry the old part to store, so that the replacement is the right size. The new T couple should be installed onto the pilot assembly as the old one was installed. The height of the old and the new element-end should be same. Gently screw the T-couple end into the valve controlling gas, to avoid thread damage. Ensure that the pilot flame is blue, without any yellow, then blow out the flame for the assembly to cool. Re-light the pilot and make sure that the light is now bright cherry red, and just 1/3 of the T-couple is heated.