I am a DIY guy, but had an issue early in winter when temperature went below -20. called this company (they installed the furnace). Didn't want to have to troubleshoot much further myself. A gentleman came in and checked my furnace. Said everything was OK (in 15 minutes or so). Left and my furnace wouldn't still work as before.. I had to diagnose myself the 4 flashes indicating the overheat (youtube) since it did't show up in that short amount of time. When called again the person told me to check the filter again(which I had changed 2 weeks before). In this process, I've noticed the old filter screens that were threre which the person that visited me failed to check. Those were totally filthy and blocking the flow. Once removed, furnace started to blow fine. Paid $100 + tax for a professional to come in and NOT notice the filthy filters in the furnace. A lot under my expectations since I didn't want to spend time diagnosing it myself. Wish I could have my money and mostly my time back.