Wonky O2 Sensor

[Mnfamilyguy2020]

I’ve been chasing an intermittent engine problem and now have a post cat O2 sensor code. Could someone help me on their location. Are they the two O2 sensors directly under the back of the engine cover?

[NorthCountryGuy]

1 & 2 are the pre cat probes. They are right on top of the ‘down pipes’.
3 & 4 are the post cat sensors and they are off to the sides.

[Mnfamilyguy2020]

Many thanks!

[Mnfamilyguy2020]

Here is a recap (long) of the issue I had and the ultimate fix which was a downstream O2 sensor:

2016 650ix Gran Coupe 42,000 Miles

I’ve owned this car for two years, it is on/a CPO.

In February when coming back from a ski trip I was attempting to pass at WOT, once I hit 80MPH the car sputtered and would no longer accelerate. I slowed down and the car continued to drive fine. No CEl or any other indication of a problem.

Once home I pulled codes and found a “super knock” code. I took the car in to my dealer, they had it for 3 days and could not recreate the issue, but recommended $1900 work - Battery, Plugs, Coils; before they would “dig in deeper.” I replaced all in an afternoon, drove the car and the exact same issue occurred. I took the car back in with my scan tool and showed them the super knock code. They had the car for three weeks, replaced the injectors, MAF’s, fuel pumps, two modules, many turbo parts, vacuum lines etc., etc. Took the car home and it sat during COVID.

I took the car out last week on a long drive and recreated the exact same issue ay 80mph. This time in addition to super knock, I had a code indication of post cat intermittent O2 Line Disconnection. Wiring looked fine, but I’ve had a similar code on a Porsche that indicated a failed sensor. I replaced both post cat sensors (thanks northcountryguy) and I immediately notice two things: butter smooth idle and a fairly significant increase in fuel mileage, also no more hiccup at 80 MPH WOT runs.