Cleaned the flame sensor. Replaced the ignitor! Replaced the gas valve. With the old gas valve, it worked some of the time, but you could see it had problems igniting. It would ignite with 1 out 5 tries. After replacing the gas valve, it does not ignite period. When it does ignite, the gas chamber where the ignitor is would be slow to fire up. The far 2 chamber would ignite first and then the third would ignite. The furnace works great when it starts up. Any ideas?