so earlier I mentioned I had a 0410 code (couldn't pass DEQ, ended up plugging in a vacuum hose and passed deq).

Today I was driving to work and my CEL came on. I returned home (I was close to home) and swapped cars.

I checked the codes and p0140 came up. That seems to indicate rear o2 sensor.

I haven't had a chance to look under the car for other potential issues (the area I work on my cars is not currently accessible).

Are there other things that would cause a p0140 code to throw?

Also, should I replace BOTH o2 sensors at the same time?
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1985 245. 246K Gone..but not forgotten. Sold in 2013, found 2018! Still running on the road. Of course. 1993 Volvo 850 soon to be gone

Does the 97 have a different O2 sensor than the rest of the 850s?

I just took mine off, and compared it to my new one, and they're two completely different lengths. And I know it's the rear one because it's the one further along on the exhaust.

BUT whats weird is, the videos I've seen show the O2 sensor being right above a plate that you unbolt (with 12 mm socket), but mine isn't. It's maybe 6 inches away from the front O2 sensor.

I did some researching (just now) and it appears there are 2 different varieties. One is much cheaper (mine is! yay!).

Just hope I can return it
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1985 245. 246K Gone..but not forgotten. Sold in 2013, found 2018! Still running on the road. Of course. 1993 Volvo 850 soon to be gone

Where did you find this car? It doesn't sound like it is USA spec. The rear O2 sensor must be behind the cat.
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Keeping it running is better than buying new

It originally was a California car.

I think I figured it out though. Much like the 93, it was part of a transition between car models. The 93 has quirks that are 93 only, and I think the 97 has some bits that are also included on the 98 S and V70s.
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1985 245. 246K Gone..but not forgotten. Sold in 2013, found 2018! Still running on the road. Of course. 1993 Volvo 850 soon to be gone

My O2 sensor came today (actually arrived 5 days early). So I should be able to tackle this in the next few days. Doesn't appear to be too difficult....

I'll try my best to update this when I'm done (I'm not the best at doing that).
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1985 245. 246K Gone..but not forgotten. Sold in 2013, found 2018! Still running on the road. Of course. 1993 Volvo 850 soon to be gone

I would replace just the rear sensor if that is indeed the problem. Since you bought the car used the front one may have already been replaced. If I recall correctly, the fronts generally last about 130-140k and usually go before the rears. On my 95 850, the front was replaced before I bought the car at 160k and the rear was original. Drove the car to 250k and interestingly never needed to replace the rear sensor....
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Will I buy another Volvo??? We'll see....

The P0140 only points to the rear O2 sensor. You could first try cleaning the connector in the engine bay and see if that helps, but if you have a lot of miles on the car I would suspect that it needs replacement.

Get an OEM replacement. Easy to identify, as it will have blue tape at all of the anchor points.

Here, in the rust prone part of the country, the most difficult part of the operation is removing the cross member under the car (4 bolts) which hides the O2 sensor. You can remove 3 of the bolts and just loosen one of the bolts on the passenger side and then slide the metal plate out of the way - makes it easier to reinstall.

If you do not clear the code, It could take a couple of engine starts to clear itself.
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Keeping it running is better than buying new

The car has 184(ish)K on it.

I wonder if this is why the car has been stammering when I start for the first time in the morning (or after a few days).

Also, I noticed that the car smelled really gassy after getting gas (I already replace the fuel filler hose).
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1985 245. 246K Gone..but not forgotten. Sold in 2013, found 2018! Still running on the road. Of course. 1993 Volvo 850 soon to be gone

Without a signal from the rear O2 sensor, the ECU cannot control the injectors properly and the engine may stumble while cold.

A gassy smell? Like raw gasoline? Make sure it is not a leak from somewhere. Usually, when the O2 sensor goes bad the exhaust smells like rotten eggs, not gasoline. Easy enough to determine: just start the cold engine, walk to the rear of the car and take a whiff.
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Keeping it running is better than buying new