question about a gas furnace having to light repeatedly
I consult to most all warm air furnace OEMs.  The best answer is it lights the first time, every time.  There are of course things that can prevent that...low gas pressure switches, low air pressure switches, limits not being made (door switch, air switch, fan speed proving, condensate switches, etc.).  That being said...EVERYTHING good...GAS supply...AIR supply...SUFFICIENT spark/ignition source (all three parts of the fire triangle), begin to look at the flame sensing.  Very early systems were single rod...hot surface ignitor glows &amp; lights flame...then switches to sensing.  This system works well..for about a year and degrades and will not sense well.  Junk them.  The two rod system has an HSI to ignite, and a flamerod to sense.  These are great.  HSI must get fully to temp to light.  If it actually "lights" the flame everytime..it is good.  The flamerod sends the signal to the flamesafeguard...which powers the gas valve and monitors all the limit switches.  Good signal..keeps valve open...heats the house.  There are specific meters to check flamesignal (DC microAmps)...but they may be difficult to read if the signal is manipulated by the flamesafeguard manufacturer (also must be read in series).  Easy fix, take a piece of very fine emory paper and clean the oxidation from the flamerod.  It will give a higher signal.  A flamerod light two or three times will help warm it up and boost the signal...hence what you have.  Oxidation will kill the signal.  There is some science to flamerods &amp; grounding area ratios.  Typically 1:4..but your OEM figures that out for you.  Make sure your green ground screw is actually grounded...this will boost your signal.  There is one somewhere near the actual burner...and one on the cabinet.  If the clean flamerod (properly located &amp; installed) fixes your problem...get a spare.  Cleaning them causes micro grooves...more surface area to oxidize.  Nobody wants to clean a $7 widget everytime the house gets cold.  Best Wishes.