Hello, its probably a flame sensing issue, resolved by cleaning the flame sensor rod off with a scotchbrite pot scrubber pad.
Look inside the cover of the unit for the wiring diagram it should have the code read out inside. If you want coaching on the trouble shooting
Please attach a well focused, close up photo of the wiring diagram glued inside the service panel, the picture needs to betaken from straight on, in very good light.
You can use the paper clip- icon at the right end of the tool bar at the top of this dialog box to attach it here. ,Include a picture of the thermostat wire connections to the printed circuit board in the same area.
If you have an error code showing, tell me what the code read out is on the wiring diagram or a sticker next to the diagram please.
If there is no paper clip-icon, look for the '+attach files' link at left of your send button.
I can mark it up with trouble shooting instructions.
You will need a multi meter ($30 at a hardware store) and some experience working with appliances, let me know a lot about your skill sets in those regards.
If I miss you tonight, I will pick up your response first thing in the morning.