, my conservative estimate is that 90% of "furnace on the fritz" issues are solved without ever having access to the physical furnace. Problems stem from electric source, propane source, thermostats, incorrect expectations, etc. In a very small percentage of cases, something like a sail switch will be the culprit.
Your thread has been viewed 89 times with no proposed answers (my furnace can be seen
when I remove the furnace return air grill under the refrigerator but it is stretch to say it can be accessed). I strongly suggest you post the details of your "fritz" so that our experts can help you solve the issue.