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These tips could easily save you a few hundred bucks.  

Background:
I've been working on furnaces since I was a kid.  That was in the 1980's and the ones that we were working on were typically up to around 30 years old so I started out working on mechanical 1950's and 1960's era furnaces.  Back then we did our own diagnostics because we had to.  The units were mechanical so they didn't self-diagnose.  

Modern furnaces are electronic and they typically have a led light that blinks.  The brand that I am most familiar with has a long pause then a series of blinks, a short pause, then another series of blinks.  The first set of blinks is the first digit of the code and the second set of blinks is the second digit of the code.  

Example:
If you go to the furnace and the led light is blinking three times then pausing then blinking two more times, that corresponds to code #32 so you look at the chart on the inside of the cover of the furnace (or look it up online) and find code #32 and it tells you what the furnace is doing.  

When a modern furnace quits, the problem is usually the igniter and if it isn't that it is usually the pressure switch.  In all the ones I've fixed I'd say it was ~70% igniters, ~25% pressure switches, and ~5% other random stuff.  

Anyway, the furnace at my mom's house quit over the weekend so I went over there to see what was wrong.  I couldn't find a blinking LED light.  I found the serial number and went online to decode it.  I learned that the furnace is a nearly 30 year old unit that is supposed to have a blinking LED light but after 30 years this one must be burnt out or else the wiring to it failed.  In any case, I had no blinking light so I had to figure it out on my own.  

Frankly, I'm REALLY out of practice at actually diagnosing these things since for the last 20+ years I've mostly just counted blinking lights, checked that against a decoder thing, and known what was wrong that way.  

The way a modern furnace works when everything is working right and how to diagnose it if it isn't working:

First off, modern furnaces are electronically controlled so like all computers the first thing to do if it isn't working is simply to reboot it.  Typically there is a switch mounted on the side of the furnace.  Shut that switch off, count to 10, turn it back on, and see what happens.  This may sound ridiculous, but I've actually fixed more than one non-functional furnace simply by doing that.  

When the furnace boots up and the thermostat is calling for heat these are the main steps of the process that the furnace goes through:

• Immediately on boot-up the furnace does a self-diagnosis check.  One part of this is to check that the pressure switch is open (more on this later).  For the non electricians/EE guys, "open" means current NOT allowed to pass.  
• After that or while that is going on most furnaces will run the blower.  This is the blower that blows the hot air out to your house not the inducer which brings in combustion air and expels exhaust gasses.  
• If the self diagnosis checks out and the blower works the next step is generally that the inducer kicks on.  The inducer, as mentioned above, is an air pump that brings in combustion air from outside the house and expels exhaust gasses back outside of the house (so that you don't get CO in the house).  The inducer is a little electric motor inside the main cover and you can see/hear it come on.  
• The inducer kicking on *SHOULD* cause the pressure switch to detect a vacuum which closes the switch.  Again for the non electricians/EE guys, closed means current IS allowed to pass.  
• Once the pressure switch indicates that vacuum is present, the furnace will turn on the igniter.  Typically these glow REALLY bright and you'll be able to see it through the combustion chamber observation window.  
• Next the furnace will open the gas valve.  Typically there is an audible "click" as the valve opens.  If everything is functioning properly, immediately after you hear the gas valve "click" open you'll hear/see ignition of the gas by the igniter in the combustion chamber.  
• The burning gas in the combustion chamber will make heat which will be spread around the house by the blower and your house will get warm.  

Diagnosis:
Like I said above, the problem is almost ALWAYS either the igniter or the pressure switch and here is how to figure out which one and then I'll give you a work-around that will enable you to operate the furnace even without the replacement part.  

If the igniter is out (like I said above, this is usually the problem):

• The furnace will boot up.  
• The blower will typically run.  
• The inducer will kick on.  
• The pressure switch will close.  
• The furnace will turn the igniter on but there will be no glow because it isn't working.  
• The gas valve will open, you should hear the click. However, there will be no ignition because the igniter isn't working.  The furnace will detect that the combustion chamber isn't heating up and shut down.  
• The house will not get warm.  

In this case, you need an igniter.  They are not terribly expensive, about $15-25 and Home Depot carries them.  Also, they usually only take a few minutes to install and can usually be done with nothing more than a screwdriver.  If you use this advice it will EASILY save you a couple hundred bucks because an HVAC guy will charge at least that just to show up at your house.  

If the pressure switch is out (like I said above, if it is NOT the igniter, this is usually the problem):

• The furnace will boot up.  
• The blower will run.  
• The inducer will kick on.  
• The pressure switch will NOT close.  
• The igniter will NOT turn on.  
• The gas valve will NOT open and you will NOT hear a "click".  
• The house will not get warm.  

In this case you need a pressure switch.  Or it is possible that the exhaust or intake pipe is obstructed.  Follow the white PVC pipes that lead to the furnace's combustion chamber to see where they go outside.  I've seen bird's nests in the pipes cause a blockage that stopped the furnace from working.  Check for that.  Ideally, check outside while the inducer is on.  You should feel suction on one pipe and feel air blowing out of the other pipe.  

If the pipes are not obstructed then the pressure switch is the most likely culprit.  Home Depot does NOT carry them.  At this point you have advanced beyond basic, Home Depot level home repair and you need to go to a furnace supplier or order it online.  

Work around for a non-functioning igniter:
If the igniter isn't working and it is the middle of the night and you can't get a new one but you want heat, remove the igniter.  It usually screws in to the combustion chamber.  Remove the screw(s), take the igniter out, and when you hear the gas valve "click" open (see above) stick a torch or gas grill lighter through that opening and light the furnace with that.  

Note that this will NOT fix the furnace.  It will only make it work one time.  Once it shuts back down, it will not restart without you manually lighting it again.  However, it will keep your house reasonably warm while you wait for a repairman or wait for Home Depot to open so you can go get an igniter.  

Work around for a non-functioning pressure switch:
The pressure switch has two wires hooked to it.  Unhook those.  They need to be disconnected when the furnace does the self-diagnostic.  Once the inducer kicks on, plug them into each other.  This will "trick" the furnace into thinking that the pressure switch is functioning properly and sensing a vacuum and the furnace will operate.  

Note that this will NOT fix the furnace.  It will only make it work one time.  Once it shuts back down, it will not restart without you doing this operation again.  However, it will keep your house reasonably warm while you wait for a repairman or wait for a pressure switch to arrive via amazon or whatever.  

Holy crap.

Honestly, I just make phone calls and write checks.

We have an air handler (new in 11/2020), which has coils in it (electric only in this part of Florida). It also serves as the blower for AC.

I think we turned the heat on for maybe a week since we've been here (since May 2020). It was January 2021 I think??

Got pretty chilly at night.

These tips could easily save you a few hundred bucks. 

Background:
I've been working on furnaces since I was a kid.  That was in the 1980's and the ones that we were working on were typically up to around 30 years old so I started out working on mechanical 1950's and 1960's era furnaces.  Back then we did our own diagnostics because we had to.  The units were mechanical so they didn't self-diagnose. 

MB you have your own residential Heating/Cooling Repair ?

I have a 12 yr old 95% eff, Bryant Furnace.A few yrs back was having problems on/off with the flame detector I think it was.Here one tech had me cleaning the wrong area but it did seem to work for a while.Thanfully I found a guy familiar with that unit he took off the incoming air PVC pipe for better access and cleaning.Good thing the installer knew what he was doing as sometimes you need access to those. So the fresh air intake didn't need to be cemented/sealed like the exhaust Line.It would have been a MOFO if they had

We built our house 30 years ago. I had a local guy from our church do the HVAC as he had a small local business and wanted to give him the work. He also was competative with the other vendors in our area.

Anyway, after we moved in and it started getting very cold, the furnace would run for a few cycles and then would not come back on for a couple hours or more. And then it would come back on and repeat the cycle. In the mean time, the temp in the house would get down in the 40's.

I called the installer and he came out and did some tests. Everything worked and the diagnositics did not reveal any issues. He went through everything he could think of and could not find any problems. He said if it happens again, call him, day or night and he would come right over and check it out. However, he had recently moved and was now about an hour away from me.

The next time it happned, I called him and he was working at another job and said that he would leave within an hour or so and head over, but by the time he got there, it was working again.

This went on for a while and was never able to get it to fail when he was here. Having worked for him a few years earlier, helping install duct work, he knew that I was capable of finding my way around the furnace. He asked if I would mind replacing a couple of parts as we had a supply store just a couple of minutes from my house that carried those parts. I agreed. He called the supply store, told them what I needed, paid for it (billed back to the furnace supplier) and I would pick them up and replace them. Most were very simple and only took a few mins. (I learned a lot about how furnaces work doing this and now have fixed my own furnace a couple of times).

But even after doing this, we still had the issue. So he looked at the weather and saw that we were going to have a very cold few days coming up and asked if he could come out and spend as long as it took to figure out the problem. I said I would be more than happy to have him there.

So a couple of days later, the temps dropped to around 10 deg and he and a technician from the manufacturer, showed up and went to the basement. About 4 hours later, they came back up and told me they found the problem.

It turns out that the exhaust PVC pipe that vented the CO2, had an elbow that was positioned directly above a vent in the furnace that was covering the circuit board that controls the furnace. This elbow had a very small crack and when it got really cold and the furnace was running more frequently than usual, it would build up condensation that would then leak through the crack and drip down onto the ciruit board and essentially, short it out. After a couple of hours, the board would dry and the furnace would work again.

They replaced the entire exhaust PVC, double checked it to ensure there were no issues, and it has worked perfectly ever since.

I was fortunate to find a guy familiar with our building and our units (water sourced heat pumps) and he's been here a few times, had to replace one unit which he said was garbage anyway, and my electric bill has gone down quite a bit since that.  Turned the heat on this morning, had AC on Sunday.

It is blustery outside.  I walked to the bank with my wife with a heavy coat.

Wow, that is a weird and random problem to have. Glad they came and figured it out. Intermittent problems are always the toughest to find.

I've learned a lot over the years about furnaces and how they work.

A couple of years ago, we got up one morning to a cold house. I have a friend that was working as a furnace tech at the time and called him to see if he could come over to take a look. He said that HIS furnace was not working either and needed to figure his out. So I decided to go through the diagnostics and figure out what was wrong.

After playing with the furnace for a few minutes and getting to understand the start cycle, I found that the exhaust blower was siezed up. I called a local HVAC company that happens to be owned by another friend of mine and told him my problem. He said give to him a few minutes and he would call me back.

A few minutes later he called and said that he located a replacement blower, but it was in Marion Oh, about an hour away from Norwalk (I live a few mins south of Norwalk). He said that everyone working for him was tied up on calls and could not send anyone until later. I said not to worry, that I would go pick it up. He asked if I could install it and I said sure, it was just a few sheet metal screws, some high temp caulk and unplugging and plugging in the wiring harness.

So I set out to Marion, picked up the part and thought, I am very close to Waldo, so I stopped by the G&R grill for a fried bologna sandwhich, which are a passion of mine. I got home a couple of hours later, replaced the blower and the furnace worked normally. I called back to the HVAC friend and thanked him for the assistance, at which time he offered me a job as a furnace tech. LOL

The friend I first called, then called and said that he finally got his furnace fixed and offered to come look at mine. When I told him what I did, he just laughed and said he should have called me to help him. LOL