Furnace model GNJ075N12C1.
Electronic ingition module: SV9500M2682

Furnace works fine most of the time... then enters the start up mode with no light off and locks out. Vacuum motor runs but no attempt at igniter glow and light off.

Seems to happen when the temperature changes on the thermostat. If I set 65 degrees and hold the temperature then all is good. If the thermostat automatically goes from 65 to 68 in the morning then it locks out.

Any ideas?