I have a Rheem Imperial drum 90 plus approx 12 years old. In the past 3 weeks there have been 4 times that the unit did not fire during call for heat. I don't know if it attempted and locked out since we noticed the house getting cooler before we knew something wasnt right. In every occurance I reset the power off and then on the unit did prepurge hsi came on then unit fired and operates ok. This happened about 3-4 years ago and 2 resistors on upper left of board were burned replaced board and problem went away. Board looks OK now, changed batteries in therm., condensate is draining. My thought is that the HSI is on its way out(original) but with just getting done with a weekend well below zero and unit constantly running had no problems. I am a serviceman with the natural gas company, I have some diagnostic knowledge but am not an expert nor do I claim to be. Any thoughts on a probable cause since the lock out doesnt repeat.