My gas furnace turns on if I reset the power to it and brings the
temperature up to set point. However, it seems that the next time the
thermostat sends the signal, the furnace doesn't start until I cycle
its power switch.
OP did not mention what model furnace. Flame is sensed by flame sensor
after ignition. It'll try 3 times and will lock up. After certain lapse
of time it'll reset itself or can be manually reset.
Most common problem with sensor is slight rust build up on the surface.
Rub it with emery cloth or such and it'll work again.
First thing, pull the error code looking at blinking LED light on the
logic control board counting long and short blinks before it is reset.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.