If it has 12 VDC but no propane it will go through 3 "try to light" cycles and then error out. In most cases it will need to be reset either at the furnace itself or at the thermostat by turning it off, then on again, in order to restart the lighting cycle again.. There may be some thermostats that will automatically reset the thermostat after some time delay.
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad