Blower fan comes on immediately. Furnace will not fire. Circuit board single flash... limit circuit open or external load on "W"

Limit Circuits are usually high limit thermostats. If it is resettable, push the button, if not resettable, replace the high limit stat. You will need a wiring diagram and possibly a parts blow up to locate the stat as well as a multimeter to see if the high limit stat is open. The W wire is the one that enables the furnace from the stat. It should be just a relay with no load on it... basically a contact switch. Perhaps that wire doesn't have enough voltage running to the furnace so it detects a "load" on it.

I'd start with the hi limit stats though.