It’s your flame sensor. It’s directly in front of one of the jets where the flame is guided into the heat exchanger. It will be held typically by one 1/4 hex drive sheet metal screw. Turn unit off, allow to cool if needed, Remove screw and sensor(unhook spade terminal) wipe it off with a isopropanol alcohol pad. Assemble the opposite of removal.

The sequence(call for heat) should go ;

Blower motor(for 90sec cooldown purge then it stops ) —> draft inducer (creates proper draft direction and negative Hg (vacuum) —> pressure switch diaphragm closes after sum check of circuit —>24vac sumcheck through rollout sensors and fan limit switches —> gas valve opens <—> igniter(glow rod) engages red hot(simultaneously sometimes) —> lastly flame sensor sumcheck of milliamperes. If motherboard CPU says “ok” then circuit stays closed til somethimg interrupts it.

Hope that made sense. Lol

Edit- ^poster is 100% correct About counting the flashes. On the inside of the service door it should have the explanation of them with a schematic diagram. I m missed the part about a new thermostat. I’m assuming it’s a fancy Wi-Fi jobber, which are nice but sometimes are not compatible or require the blue wire/common to be hard wired to the unit. Most standard thermostat do not require this however, and it can cause issues. Quick way to rule out the thermostat is to take the white wire and the red wire and manually call for the heat by jumping them together.

https://imgur.com/a/GP0OzNE

Any idea where the sensor is?

Top middle. Single black awg wire w/red female spade on it.

Rollout switch might be tripped . You should see a small window on the door of the furnace. If you look in there and see a red light, count the number of times it blinks. Sometimes the panel will have a guide that wil tell you how many blinks means what. You can also google make model and blinking light

Ignitor could be bad too, yes?

OP, when the furnace kicks on, do you see the ignitor glowing, and flames being lit?

Yes, the igniter is next up. If it doesn’t glow there is no call to the gas valve. It will cycle three times trying to light. On the 3rd failed attempt it typically shuts off for anywhere from 4-6 hours or Until reset, Depending on the dip switches respectively. You can Test the board where speed terminals connect the igniter for voltage ; Or if you don’t have a voltmeter unscrew the screw, remove the igniter and plug it up to 110 power source/wall receptacle to see if it glows. Be careful, AND DO NOT TOUCH THE CARBON ROD OF THE IGNITER WITHOUT LATEX OR NITRIL GLOVES. The oil from your skin/fingers will deposit onto the igniter and it will cause it to fail and fracture next time it lights up. If it works that is lol.

This is exactly the problem mine had as well. I had to have a tech come and diagnose this and they helpfully explained what this post covers. It’s a fairly simple system if you’re comfortable with a couple of tools.

You rock. Great writeup.

Helpful video

https://www.youtube.com/watch?v=tYwR9qL4lnk

🙏 Was a maintenance super for about 1200 units spread across a city of about 700k. I could usually talk most residents through simple repairs over the phone. But always came if they didn’t feel comfortable….lol. either way ima get the designated hour alloted and close out the order!

make sure the gas line is actually open hehe

I had this problem and it was the igniter. The previous owner of my house actually had told me this had happened to him once, and he told me he left me a spare igniter and showed me how to replace it. I took a video of him showing me, that I ended up needing to reference just a few years later. I went the next day and bought a new spare igniter and I’ll always have one on hand because of this.

Yup. This is how you homeowner!

I've replaced mine twice in the 13 years we've owned the house.

Turns out it was a high limit sensor that wasn't resetting.