T-stat calls for heat........furnace fan comes ON. IT runs to purge combustion chamber first. The blower causes the 'sail switch to close so that 12V power can go thru sail switch, thru high temp limit switch to control board.
When purge time is over..power from control board goes to gas valve solenoids opening the gas valve and at same time the spark electrode is energized. The spark ignites the gas and then a signal via the spark electrode generated by the main flame goes back to control board to prove the main flame is lit so power stays on gas valve holding it open. When t-stat set point is satisfied t-stat opens...power drops out to control board..gas valve closes and fan blower runs just long enough to cool/purge combustion chamber.
SO.......
No spark. Could be sail switch---they get dirty and don't move/make up so no power to board.
If not sail switch then could be control board....the connector on board could have poor contacts and needs cleaned
If it sparks but main flame doesn't light
Could be....spark electrode is out of position
Here is a service manual that covers your model.
Page 16 Sequence of Operation
Page 22-28 Description of Components
Page 29 Troubleshooting Guide
LINKHere is a 'less technical' troubleshooting guide with pictures.....
LINK Probably needs a good cleaning...sail switch, board connector contacts
