I'm servicing a Lennox GCS3-953-250a-7 (manufactured in 1972!) for ignition failure (CO readings tested by gas company a few months ago; unit is safe to run). The inducer comes on and closes the proving switch. The ignition control board energizes and the ignitor is sparking but the control board never sends voltage to energize the gas valve. My first thought was a bad board, but after swapping the board with a known good board (that functions just fine in the unit next to it) I can eliminate the board.

This unit is located in a local convention center where inexperienced $9 an hour in-house maintenance has "repaired" it for the last forty years and with an unreadable schematic I'm at a loss as to how the controls are supposed to be wired. I could spend all day following each wire to reverse engineer a schematic, but I'm hoping somebody here will have access to a schematic. The reset commonly found behind the blower access panel has been bypassed and removed at some point.

Also, any experience with this would certainly be appreciated. I only got about 45 minutes to look at it before a heavy rain started and forced me off the roof. I'm returning tomorrow and would appreciate any wisdom you may have picked up on these units over the years.