1998 Dodge Ram SST Magnum 5.9L has 152K on it. No CEL but needs upstream O2 sensor.
I have either read or heard that Bosch O2 sensors don't function quite right on these engines.
Anyone have experience with Denso or any other brand? I avoid cheap Amazon made you know where parts.
Should I be paying for genuine MOPAR O2?

On a 1995 Ram 5.9 Magnum V8 the Mopar OEM O2 sensor was made by NTK, which is a sister company to sparkplug maker NGK.

I ran a Bosch O2 sensor for awhile but it stopped working after about 30,000 miles.

I remember posts by Texas engine builder Larry Widmer who stated that on Hondas, a Bosch O2 sensor would create a 14.4 air to fuel ratio whereas an NTK O2 sensor create more like 14.7

When it comes to sensors for our Mopars you better buy the Mopar brand sensor cause the aftermarket ones won't last a year.

With no light or codes why do you want to replace the sensor?

My 2002 Durango had sensors with both the Mopar part number and the NTK number on them. One was USA made and the other was made in Japan.

Truck starts right up and idles well in open loop for about 1.5 minutes.
When it goes into closed loop it idles rough and if I drive it pops through throttle body indicating a lean condition.
This points to upstream O2 sensor, manifold incoming air temperature and possibly to coolant temperature. Not 100% on the last one.
Once warmed up, all is well.
I did cover all the basics, spark plugs, wires, cap, rotor. I have replaced the IAC, MAP and TPS when I had throttle body off for cleaning.

Have you checked the intake lower plenum for leakage. If the o-2 was bad it would throw a code. You have another issue causing what is going on.

Good question regarding the intake plenum. A known issue.
I have taken care of that. I bought the improved plate and gasket from Hughes Engines.
Another one: Fuel pressure regulator. Fuel pump assembly replaced about 3 years ago. I recently checked fuel pressure.

I would scan the computers coolant sensor as it is warming up. It could jump indicating a warm engine so the computer will go lean when you need more fuel.

^^^^ This makes the most sense to me after dealing with "are the O2 sensors my problem" on the wife's T&C and the hungry groundhog. If all is fine after the engine warms up then.................

If the price of Wide Band sensors would come down just a bit more, the above situations would justify sticking a wide band sensor on an extension pipe at the exhaust tip. I guess one could weld in a bung in order to temporarily move the after Cat narrow band oxygen sensor in front of the cat.

When buying OEM you may get a better price from a Dodge dealer online as compared to your local Dealer. Sometime Rock Auto sells Mopar brand parts as well.
Example from RockAuto: https://www.rockauto.com/en/moreinfo.php?pk=5099253&cc=1314573&pt=5132&jsn=1350

i have seen coolant sensors turn green on the bulb that is in the coolant flow.
when this happens, it seems the resistance values go wonky, thus scanning in real time could give you a solid clue as to what is going on.
if you remove yours and it is tarnished, shine it up with a scotchbrite pad or steel wool and see if that makes it work better.
that has worked for me many times, and you really have nothing to loose at this point.

NTK brand O2 sensors are way better than most others, cost more also

and to throw "stuff" out there...............
my 99 caravan [known as "the bus"] needed a downstream O2 sensor a couple of years back, and of course it was not stocked locally.
for some reason, i had two NTK sensors on the shelf, and the box numbers coded out that they were for a ford taurus. i don't know why they were there, but i never through ANYTHING out if it can POSSIBLY be used for something !
anyway, the wires [4-it was a heated sensor] matched the bad sensor on my bus, and were in the same order in the plug. however, the plug wasn't the same as the plug that was on the bad bus sensor.
after looking at both sensors for a bit, it became apparent i could swap the plugs, and use the ford sensor on my bus, so why not give it a try ?
so i popped the wires out of both plugs and snapped the ford sensor wires in the bus's plug.[in the correct orientation of course] installed the ford sensor and it WORKED !
it's been in the bus now with no problems for several years, works as it should, and no codes set.
i couldn't believe it would work, but it has, and i have an extra one [remember, i had two on the shelf] on the shelf if i need it.
i guess trying something just because, may be a great learning experience, especially if it works !

Installed probally a few hundred at the shop Most weer Bosch They work period In all we used never had a faliure and most cars were abused and driven for years.Rocky

How do you know that?

Not necessarily, You did the plenum but did you use fel pro gaskets? The Hughes ones are junk. When did this issue start? Also what plugs are you running?

So folks, problem was the O2 sensor after all. I replaced it with an NTK from NAPA. Runs perfect now.
It is rather puzzling why it did not throw a CEL.

BTW, i did use Felpro gasket instead of the Hughes Engines supplied gaskets for the intake manifold kit. That was probably 5 years ago.
Last week I installed a Milodon high flow water pump on my 1973 Charger. There too I tossed the supplied gasket and used Felpro.

Thank you all for the good advice. Great forum.