Furnace is a coleman, owner had our tech shut off the gass or lp gas after he installed a new thermal couple .now the furnace will not light, I only get pilot flame and when I release pilot flame dwindles down and goes out . Plenty of propane exist .I suspect the valve might be faulty or the preasure regulater in it. What is common here? Unit is 15 years old.

Hold the button in longer, if it is a new t-coupler the only other thing it could be is a dirty piolit orfice casing the flame not to hit the t-coupler