Our gas fireplace would just not work.
The pilot light was not staying on -unless I was keeping the ignition knob depressed- no matter what I was doing. I cleaned the oxygen sensor and I repositioned the logs. I was so mystified that I even called a gas fireplace expert who told me that I needed a new thermocouple (which he would 'love to install for me').
Well, I just did not believe the gas 'expert', even though I paid him $85 for 1 hour of his trials and tribulations.
Immediately after he left, I noticed that the pilot light was not touching the thermocouple shaft due to it being too low relative to where the pilot flame was hitting. It just occurred to me that the thermocouple could not engage the valve to keep the pilot on. (Without the pilot light on the fireplace cannot be lit, for safety.)
So, just as a last resort before buying a new thermocouple, I attached a flashlight to my head, took a Phillips screwdriver, stuck my head into the fireplace, and disconnected the small metal brace that keeps the thermocouple attached to the burner -it also keeps the piezo element close to the pilot gas tube- and repositioned the thermocouple so that its tip was about 1/2 inch above the pilot flame opening.
The interesting thing is that the thermocouple has a circular groove around the base that is supposed to indicate how is it to be 'seated' on the support and for some reason it slipped from its groove. (So I just repositioned the thermocouple on the proper groove.)
I reattached the plate -together with the piezo element next to the pilot burner- and, viola, the pilot flame stays on after just 6-8 seconds of keeping the ignition button depressed. It only took me a grand total of about 10 minutes of work -plus a few head bumps on the flue damper- and that includes the removal of the logs and their repositioning at the end.
When the pilot flame is on it hits the thermocouple at all times and now the fireplace works like a charm. I feel terribly sorry I did not spend the $85 I paid to the gas 'professional' on a few good bottles of Shiraz or Zinfandel.
Next time I will definitely do my homework before I call a pro...
As far as how much of the thermocouple should be in the pilot's flame path, have a look at the image to the left below. In my case, before I corrected the problem, the tip of the thermocouple was about 1/2 inch below the pilot burner flame axis. Go figure!