I have a 98 Ranger with 3.0L V6.
Lately when I have been driving it, it will sometimes suddenly lose power like I've flipped the key off for half a second. Then it will pick back up and run fine. This normally happens after a cold start or it has been sitting for a few hours. After it does this, it will run fine.

Additionally, I've been getting repeated P0174 - System too lean (Bank 2). That is the ONLY code and has always been the only code. I can clear the code and drive it fine without the CEL coming back on for weeks or maybe months. Then suddenly, without any noticeable symptoms, the CEL will pop on again. Same code. Sometimes it idles a tad rough, but not terrible. One time after driving it a long distance to pick up some equipment and I was almost home, got to the stop light at the offramp and it idled really rough. Slowly lost RPM and it died as if I was low on gas, but I wasn't. Started right back up fine. It has not done this again so far.

Also, when it is cold, it will start up rough for a second or so, but once it picks up, it runs fine.

 

Codes like P0174 need repeated "lean"(higher STFT) incidents before computer will set a code, so unless there is a steady high STFT(vacuum leak) on that bank it can take awhile to come back

O2 sensors last 100k miles or 12 years, which ever comes first, if yours have not been changed then I would do that

1998/99 Rangers were first years for Returnless fuel system, actually the ONLY years, lol
Ford put a fuel pressure regulator(FPR) in the gas tank on the OUT hose of the Fuel Pump, this didn't work out well, it was OK but fuel pressure could go out of spec, 50-60psi, under some driving situations
In 2000/01(and up) they added a Return line on the Fuel Filter, 3 port filter, and FPR in the tank was on that hose instead of fuel pump OUT, this made for more predictable fuel pressure in all driving conditions
There is no Fuel Pressure sensor, so computer can't react ahead of time to a drop or spike in fuel pressure

On cold start try cycling key on and off 3 times and then start engine, see if that eliminates the rough running just after start up
If so, your fuel pressure is dropping to 0psi when vehicle sits more than a few hours, it should hold above 25psi for MONTHS and MONTHS
Each on and off of the key adds 10psi of fuel pressure to the system, so start up with just 10psi can cause rough running until pressure builds up once fuel pump is on full time

 

So this truck has over 200k on it, so I'm definitely going to need to try some new O2 sensors. The question now is, which one(s)? Probably all, I'm assuming. Look at RockAuto's options for Daily Driver, I assume I'd need both upstream left and right, and one downstream. Is that correct? I may as well get some new spark plugs while I'm at it.
Are the O2 sensors fairly accessible for me to replace or would I have to take off other things?

I'll try the key cycling on/off next time I need the drive it.

https://www.rockauto.com/en/catalog/...2)+sensor,5132

 

1998 V6 will for sure have 3, and can have 4, they are all the same kind of O2 sensor, only difference is the length of the wires attached, the lead, and of course the Connector, Ford O2

Yes, all are over 12 years old, so change them all
Your MPG will get better regardless

Nothing needs to be removed but they can be hard to access with regular wrenches
They use 22mm or 7/8 wrenches, either will fit

Since there are wires attached you need to uplug the sensor then thread the wire thru a box wrench or use a flare nut wrench, an open end wrench can just round off a rusted sensor's nut

They can come out easy or be rusted in place, you won't know until you try and one can be easy and other hard, or visa versa

Do one at a time, do not mix up connectors in engine bay
Had a shop do that to mine when replacing the clutch
Lean on one bank and rich on the other after warm up, lol, took me a few hours to figure that one out, shop, of course, said it was nothing they did