And when I mean quick, I mean smoking fast. If you have any fireplace that uses a millivolt generator to keep the burner on, and the flames go on and off, you may want to read this. A millivolt generator is just a fancy term for a big thermocouple or thermopile. Heat from the pilot burner flame touches the generator and makes about five to six hundred millivolts, or about a half a volt. This is supposed to open the main gas valve when the wall switch to your fireplace is turned on. After about ten years or so, the pilot burner and generator wear out and may not work reliably. If the volts generated are less than 500 millivolts with the switch off, then you need to replace the pilot assembly with the thermopile included. Don't try and replace these separate as these parts can break trying to get them out anyhow.
If you've done this, or you are already getting at least 500 millivolts, then you might want to look at the switch on the wall (I would still replace these parts if over ten years old as heat will weaken them). Frequently, builders will use a 120 volt wall switch to operate the low voltage. The switches cost the builder or installer less than fifty cents and will do the job, for a time. The problem comes when the switch starts to wear out a bit. For 120 volts, this isn't an issue because the force of the electricity will overcome the wear for decades. The very low pressure from the millivolt generator won't be able to overcome the resistance of this switch and the result is your fireplace is going to go on and off quite a bit. The fireplace manufacturer has a switch that is supposed to be used and it's designed for low voltage operation. It's also over $30 to $40 for the switch wholesale if you can get it. However if you want to have your fireplace working right you need to use this part and not cheap out. Maranatha!