I turned on the heat yesterday for the first time this fall, and the furnace came on and blew warm air for awhile, then only started blowing cold air. The blower kept running blowing out cold air, so I turned on air conditioning to get the air stopped, waited a few minutes, then turned on the heat again and the furnace started blowing warm air again. My furnace will blow heat for awhile, then only blows cold. I think the thermostat is faulty. Would that be a correct analysis of my problem? Thank you for your response.

I turned on the heat yesterday for the first time this fall, and the furnace came on and blew warm air for awhile, then only started blowing cold air. The blower kept running blowing out cold air, so I turned on air conditioning to get the air stopped, waited a few minutes, then turned on the heat again and the furnace started blowing warm air again. My furnace will blow heat for awhile, then only blows cold. I think the thermostat is faulty. Would that be a correct analysis of my problem? Thank you for your response.
It sounds like to me that it is possibly the flame sensor. Depending on the year of the unit if it has a pilot or an ignitor.

I go along with the flame sensor, but for to check the thermostat see https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html