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Most likely if it had not been used in a while or the LP tanks were changed the furnace LP line still had air in it..
Normal sequence of events will always be.
1. Blower motor turns on and purges for 45 seconds
2. Sail switch moves to complete circuit via airflow from blower
3. Igniter fires and LP valve on furnace opens
4. Light off occurs.. furnace runs till set point reached
5. LP valve closes, furnace flame off, blower motor runs for 30 seconds to cool the furnace combustion section
IF light off doesn't occur the sequence of ignition retries 3 times and if not flame detected then it goes into lockout mode
To clear lockout mode turn furnace OFF and back ON.. sequence above will start again
Another weak area is the (Black ATC) style looking fuse often used on the Dometic/Atwood furnace.. This is not a standard fuse.. it is a resettable fuse that often doesn't function correctly and can give problems
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