ComfortMaker GUH075 series high-efficiency (93%) furnace isn’t making heat

This isn't a great answer but I want to provide something in case it helps others.

There was no issue with my flame sensor or the ignition device. It went on just fine but was kicking off due to some other reason.

After a lot of troubleshooting it was fixed by one of three reasons:

• powering off the furnace for a couple hours with thermostat disconnected.

• installing new thermostat. I was due for a new thermostat and not sure what could have been wrong with the old one. It did run out of batteries right before this but it doesn't make sense that when I left the old thermostat it worked fine with the furnace cover off. The only thing I can think of is the old thermostat was sending incorrect signals to the furnace that caused it to need more air during usage (maybe an AC expert might be able to give a plausible reason for this).

• Reseating cover with slight gap. I have since put this fully on but left it with a slight gap the first couple of weeks.

I did nothing else to the unit and the unit was in otherwise pristine condition - only a couple years old.

I agree with the response above it is imperative that your gas pressure be right. Manifold pressure is 3.5" in our area. Although if you pressure was significantly down I doubt if any of the burners would light. Let me tell you what I ran into one time. I basically had the same thing, three burners would light just fine but nothing on the fourth. I checked my pressure, all good. It was also the first start-up in the fall. I pulled the manifold out and started looking at orifices. I was checking the bore on the orifices. I looked into the #4 orifice and I saw what looked like a white film. Now I'm totally scratching my head. I unscrew the #4 orifice. Out comes a family of I mean tiny spiders. I had never seen spiders so small, like half the size of a period on the end of a sentence. They were so small that they could easily crawl through the orifice. The white film was actually spider web. They had webbed across the orifice opening from the inside. The 3.5" of gas pressure was not enough to break through the web. It totally blocked the gas. I asked the customer for a Que-tip, swabbed out the opening, put it all back together and away she went. Good Luck!