Do you have a crawl space & have you investigated there if you do?

I have no crawlspace. The bottom of the stairs is the first floor living area and the top of the stairs is the 2nd floor bedrooms. The basement area doesn't have this smell and there is a dehumidifier that keeps it at 40%

Smell the black treads on the stairs and see if they are guilty of the odor.

thats what I thought of but the smell was there before those were put on.

Check switches and outlets in the area. Turn off the breaker, remove the cover and check for warmth or evidence of melting. Some of the plastics they use can give off an odor when a bad connection causes them to heat up.

Already did that.

Is that the front door at the bottom of the stairs?

yes

Do you have air returns or ducts? You might have them cleaned.

No I have baseboard.

Have you sniffed around the threshold of the entry door? What does the exterior side look like? Is the door under a porch, etc? Do you have a storm door?

So I cleaned the threshold and checked the wood and sniffed around that area and doesn't smell. It's just a set of concrete 3x3 porch to steps.

Is there any way to access the under stair area or is it all enclosed? Smells like that drive me crazy and if you can't get under the stairs, I'd investigate the cheapest way to access it (removing a couple of treads, cutting into the drywall) and see what's under there.

I supposed that the previous owner could have spilled something on the stairs, but it looks like you've painted and that could seal something like that so it doesn't smell. Did you use a primer/sealer on the stairs before painting? If it was something sour that seeped into the wood, it would seem to me that the smell would be worse in the warmth of the house in winter.

Wishing you luck for figuring it out. If you do, please come back and post.

Looks high & dry so we can probably safely rule out water intrusion. I second what Olychick suggested--sniffing around underneath the stairs.

Yeah the basement stairs are right underneath the main floor stairs and the basement stairs ceiling doesn't look to have any damage or smell. So im thinking I should just drill a hole in the main floor stairs and see if I can see anything.

Maybe invest in an endoscope to see into that hole you're drilling?

https://www.amazon.com/s?k=endoscope+camera&crid=1QVS2SFHOX8ET&sprefix=endoscope%2Caps%2C664&ref=nb_sb_ss_i_4_9

Also can you describe the smell? Like mold or ??

I say a mild sourness? Its not damp smelling or musty. Maybe a stale smell.

If there's nothing obvious from a drill hole I'd agree with Olychick and consider cutting into that drywall on the right as you face the stairs. I once made that area into an extra storage place by putting a door on it so might be an opportunity.

Years ago when we built a house, the contractor would leave kind of a mess under the stairs to the upstairs (before they were enclosed below). I made him promise he'd clean it all out before he enclosed it but I was never sure he did. It was just sawdust and little bits of wood and drywall scraps...no garbage or anything. It always kind of bugged me that I never saw it before he enclosed it, though he was a good guy, so I should have just believed him. But I always wondered what anyone remodeling or tearing down that house would find under there. Not sure how old your house or remodel is, but it's possible something is under there.

Built in 1976, remodeled in 2016, and we bought in 2019.

What kind of flooring is in the entry? Nail down wood, glue down engineered, vinyl? Is this flooring only in the entry?

Since the house has been remodeled fair recently, I wonder if the PO might have installed flooring over old stink--like pee stains?

Is the roof new? On hot days could it be the smell of tar?

The flooring is a floating pergo type flooring. I would understand if there was pee stain floor underneath because upstairs was the same. I had to ripe out the carpet, put a sealer on the subfloor and put life proof vinyl planks down. I dont think thats the smell though. Im fairly familiar with that smell.

Also not the smell of tar.

We have 3 Austin Air Purifiers that we can start using again after we replace the extremely expensive filters. Before we run those again my wife bought an enzyme cleaner that we are going to use on the floors and walls. If that doesn't work then my wife mentioned something about an ozone generator. I have to research more but that may be an option. Also we boiled vinegar for an hour a few days ago and the smell is still there but it seems like it smells different now So im still at square one but still have some things to try.

Ozone or any other treatment will only be a permanent fix if there is a limited source that will be destroyed once and for all. If it's being generated from somewhere, like a leaky roof, mold etc. it will come back as soon as you remove the generator and reoccupy the house.

If vinegar affects it you can leave some dishes of it around (in places where it won't get spilled) to passively suck it up.

Are those filters activated carbon? Do they no longer remove the odor or is there just a timer on the machine telling you it's time to replace? They make work longer than 'rated' for.

any update on this? i have similar situation in the staircase

Have any of the walls been painted recently? I had some fairly new cans of paint that soured once. i tried some iof it on a test board to see if it still smelled sour when it dried and it did