I have the very popular Laars LLD heater. Until today it worked fine since I last replaced some parts 2 yrs ago. But today it does not ignite. The igniter glows - three cycles. And there's the usual click sound while the glow is at its peak. But no ignition and no gas odor. The 500 gal. propane tank is nearly full and the valves are both in on position, of course. I always thought that click was the gas valve turning on, but no gas flows. Could it be the flame sensor - assuming that sensor also senses when the igniter is up to proper temperature?