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Hi I'm Brian and I'm here to help!
You've introduced air into your gas lines and short of purging the gas lines to your furnace, which should ONLY be done by a professional!, You'll have to cycle the heat on your furnace and if it locks out before it fires off and runs, you'll have to turn power off to your furnace for 30 seconds or so and turn it back on and cycle the furnace again. You'll have to do this as many times as it takes until your furnace fires and continues to run.
Give that a try and let me know how it goes.
This is your original question and should be the only one you're charged for. Please follow my instructions above to purge your furnace gas line of air and get your furnace heating again!
Your furnace should have a safety shut off switch mounted either on or right near the furnace. It may look Like This or it may look like a standard light switch in a box mounted next to or on your furnace. That is what you turn off for 30 seconds then back on. Do this if your furnace locks out due to no flame sensed because of the air in your gas line from the propane running out. It may take 3 or 4 times to purge the air (or more depending on the length of your gas line).
Make sure you have your thermostat set to heat and high enough for the furnace to run during this whole process!
It's hard to say. Like I said above, you may run the furnace, have it lock out due to no flame sense, reset it by power off/on, furnace restarts and it will either fire or it will lock out again. How many times you have to do this depends on how long the gas line is and how much air got into it which neither of us has any way of knowing.
Once you have gas to your furnace and under normal conditions, the intermittent automatic pilot should light within a couple of seconds when the gas valve's pilot valve is opened. How long it takes from power up or first call for heat depends on the furnace model itself. When heat is first called for, your inducer motor is turned on and some furnaces have a pre-purge that may last 45 seconds before the pilot is called to light. It just depends on the furnace. It could take a couple minutes of running (with no fire) before it locks out. This is a function of the control board in the furnace and is different depending on furnace model and control board model.
As I said earlier, it could take 3 or 4 lockout cycles or it could take more. I have no way of knowing that. The only way to get it running is to purge the air out of the lines and the only means you have of doing that is to cycle the furnace through as many lockout cycles as it takes to get the air out. As a tech, we would purge the gas line manually but that is not an option open to those who are not trained in servicing gas appliances!