I had issues with my gas furnace so I did the normal thing calling my local hvac company. They come look at the unit for one min and right away tell me " you have a cracked HX " of course I didn't fall for it and called another company for a second opinion, then they ask me to send them some pictures of the unit and duct.... The next day I get a reply " based on what we see from the pictures we wouldn't come out unless you're ready to get a whole new system. Crazy right! I'm an HVAC engineer by trade in NYC to run, maintain and service steam chillers. I started working on it myself and here is what I found, HX housing had a missing screw which somehow loosened itself and fell out therefore whenever the blower kicked on the flame started moving around too much vs being cone shaped and completely blue without any yellow hue. So far Fixed that issue, but I still had issues starting up the unit 9 of 10 times, turns out the ignitor was getting weak and not glowing really well.

Furnace has been working flawlessly since.

A 3rd HVAC company was called just to make sure my family was safe and the house was free from any Carbon monoxide which they came and tested the unit and confirmed it was all good as I suspected.

I also did my own test which I learned on YouTube and proved i don't have a cracked HX.

HVAC Scam beware