My 2004 Honda Pilot with 185k miles might have a transmission problem but my mechanic and I are unsure. For the last month it’s been throwing a code on and off for the o2 sensor behind the cat converter. Today I was driving it around 30/45 miles per hour. I could feel it hesitate as it shifted gear so i pulled over worried maybe the oil got too low as it’s been burning oil for the last 3,000 ish miles. Since my last oil change. It was low so i added oil. Continued on my way. Came to a stop and could feel my idle was off. Just felt a little shaky or maybe stuttery. So i put it in Park to see if it would smooth out. It did. So i shifted it back into Drive and all i got was a reved engine and a slight roll backward like it was in nuetral. I used that to back into a driveway, i’m not sure that reverse worked at that moment but after i was safetly off the road reverse and drive were giving me nothing. Then i sat and let it run for a minute no change. Then i decided to shut it off and turn it back on. Sure enough reverse and drive worked again. I made it about a mile and then when i hit the gas it stopped automatic shifting and the check engine light came back on (still coding for the o2 sensor no transmission codes) and just reved so i parked it again. Worked fine to pull it on the tow truck. (Also have not seen the flashing D light that i’ve read about.) Could it just be the o2 sensor just is completely gone or is it the transmission. I’ve only owned it 30,000 of it’s miles so not sure on it’s maintenance but the person i bought it from’s brother worked for Honda so she said they took good care of it. My mechanic said the transmisson fluid is black and smells wrong. Could it just need a change? It has all it’s fluid is not low in any way. Sorry this was a lot to read thank you in advance.
Talk about a waste of time typing all this, and not once indicating what code(s) where pulled.
???
Tester
My mechanic , Maybe that needs to be your former mechanic . Me thinks you need a transmission shop to look at this ( not a chain type or dealer but an independent shop ) . Also Auto Zone might read the codes so you can post them here and get better suggestions .
P0157 is the code. Sorry i didn’t know you’d need it when i said it was throwing the code for an o2 sensor.
Sorry i tried to be as detailed as possible i didn’t know you’d need to know the code P0157.
That code indicates the voltage from the O2 sensor after the cat is too low.
Probably a bad O2 sensor.
The O2 sensor after the cat has nothing to do with how the transmission shifts.
Tester
The following would seem to prove otherwise:
Yes, you can try a fluid change, but if the trans fluid is black, it is very likely that the trans is on its way to dying.
Just for the record, trans fluid and filter should be changed every 3 yrs/ 30k miles. So, by my math, the trans fluid should have been changed 6 times, so far.
Unless the previous owner (and you, who has apparently driven it for 30k miles) adhered to that type of schedule, I am not optimistic that a fluid change at this point will help.
I agree with VDCdriver and black transmission fluid is not a good sign. That means the fluid is oxidized due to slippage in the clutch packs and clutch material mixing with the fluid. Change the fluid and maybe make the sign of the cross. I think the O2 sensor is the least of the worries at this point.
Some oft used but seldom applicable phrases are “took good care of it”, “maintained it religiously”, and “it’s in excellent condition”. The last one is used a lot when someone is trying to sell a car… 
Yup!
Any claims of good/excellent maintenance need to be taken with a very large grain of salt unless there are hard copies of the maintenance record to back-up those claims.
Another line that people used to use was, “It was only driven to church on Sunday by a little old lady”.
As if that type of severe service was a good thing…
In other contexts, some oft-used lies include:
The check is in the mail
Of course, I’ll still respect you in the morning
and…
Of course, I won’t xxxxxxxxxxxxx (never mind, I won’t finish that thought, but most guys will know what that was leading up to.) 
+1
I wouldn’t advise that the OP spend any money on the possible O2 sensor issue until a VERY expensive transmission overhaul has been ruled-out.
Yep, my engine runs funny, I add oil.
I zeroed in on the “black transmission fluid” comments. Not good, Bob, not good.
You might try a transmission fluid and filter change. It couldn’t hurt. But methinks a new transmission is in your future.
We had a 2003 Odyssey that I suspect uses the same transmission. When it went out on us, quotes were around $2500 to repair/replace it.
Maybe that helps, but good luck.