We have a Carrier furnace that is 5 years old, the thermostat is set for 23 degrees, and the temperature is sitting at 19 degrees. The furnace quite often will not kick in, if I open the door of the furnace and manually hold the switch it sometimes kicks in with the first try; however I normally have to depress the switch 2 or 3 times for the furnace to kick in and stay running. Can you tell me if you think the problem is in the switch or in the thermostat? And how would I repair this myself?

Clean the flame senser