Why is my Lennox g40uh series furnace not turning on?
Sounds like you have most of the common bases covered.  This happened to me and it was the blocked inlet valve, a leaf had gotten sucked in.  It took me a while to find it, it was not in the visible area of the inlet, it was stuck on a screen to catch debris.  I would double check that.  Maybe try to blow some air in the inlet from the outside of the house and see if you feel the movement in the furnace.  

This question is similar and may have some other suggestions.