2008 Yukon Denali vey rough misfire out of nowhere

[B-train]

Before you do any work on it, know how much ethanol is in your fuel and compare that to what a scanner reads for alcohol content. If they don't match within a few percentage points, you may want to consider resetting the alcohol content (unless you're actually running E85).

I don't believe they have an ethanol sensor. From what I've seen with HP Tuners on my 2008 in the fuel tables, there's nothing there as far as measured content. I think it's all based off the O2 feedback.

I don't recall any modifier tables, etc for ethanol on the 2007 and 2008 with the 6.2L. I have run into the lean codes occasionally from running too much E85, or straight E85 and the injectors can't spray enough fuel to keep the O2 sensors happy. That being said, it never ran any different and was an animal on straight E85

 

[Marky Dissod]

These motors can run fine on 87, but definitely like premium or better due to the compression ratio.

These motors can WALK, or drive EASY on 87, but if you want to make your Denali play or work HARD,

... make a blend of 25% to 30% of total capacity (I go by the fuel used counter on the DIC),
and that will give you 93-97 octane.

Although this may not be as effective in winter, because 'E85' is actually E51 once it gets cold enough,
this is still a great idea if E85's price is at least 20% less than 87.
In my case, E85 is only $2.90 whereas 87 is $3.00 ...

Guessing they sell 85 where you are because of unusually high altitude?

My thought was that if I was having a misfire, it would cause the O2 sensor to be off.
I do agree with you that it would be an easy step just to replace the O2 sensor and see what happens.

If it would be easier for you to switch the driver side injectors with the right side ones,
you can do that (hopefully for FREE?), and see if the codes also switch sides.