Checking with Datamaster from TTS I found that my front O2 sensor was not responding, so I bought and installed a new one- same thing, unresponsive- rear sensor stays at 5100mv while front O2 sensor responds normally. Connection is good, no frayed wires, What else could it be? I'm not getting any DTC's.

 

For ***** and giggles... swap the two O2s.

Another thing would be to NOT wire tie that O2 between the sensor and the plug and see if that helps. ( O2 sensors need to be able to sample outside air, it does this thru the wiring harness between the sensor and the plug. This is called 'reference air' and an O2 sensor makes voltage from the difference between the two 'airs'.).

a 2010 Fatboy still has the big 18mm sensors, right?

Another thing is the bike can be so rich, the O2s fail to work right. This could really happen if you did a VTune with a marginal sensor and then the sensor took a poop. New sensor gets flooded right now from the bad AFVs from the bad sensor.

For a test, alter the VEs on that cylinder by like 20% for a quick check. You will wish to DECREASE the VEs. You want to lean out that cylinder for the test.

 

I'm the kind of guy that will go so far to look for my missing shoe under the fridge just to rule it out, so I gave it a whirl, and it remain a front sensor issue. When I switched the front to the rear it worked fine, but the one previously working I the rear became unresponsive.
I decreased the VEs, no change, tried a different calibration, no change, I stuck a rabbits foot in my pocket, no change.
Next? I'd hate to have to bring it in and pay for service. I should have bought the Road Glide I wanted a few months ago, but it's such a cool bike and I've put a lot into it. Heck, I was even seriously going to a 107 or 117 kit from Hillside this winter.

 

I really like Touring bikes, but that's a nice looking Softie there.

Well, crap. How long did you leave that other calibration in there? Totally different from the one you run, right? If it is O2 flooding, it would take a bit to clear out the Sensor.

And... the other sensor falls right down in Datamaster as it heats up, right?

Your bike has 18mm sensors right, IIRC, 2010 Softies did.

I'm STILL thinking sensor flooding and really wish for you to try taking 30% off of the VEs and wait a bit to see if the sensor clears. Just highlight a portion of the VEs, make a box you will be changing, then up top use the percentage button set at 30 and click em down one. Run the bike in the area you lowered the VEs for a bit, like idle...

But... now I am starting to also think wiring. My problem will be is I have no manual for softies. Can you get your hands on a decent digital multimeter? I like Fluke meters myself. They are decent mid range stuff.

Look at your manual, and tell me if you can figure out how the wiring for that SET of O2s is run from the ECM to the each sensor, OK. And... if you haven't done so, unplug the ECM from the harness and take a good light and magnifier and look at the pins on the ECM and also on the Wiring end for ANYTHING that looks suspicious.

I'm going to think a bit on this while I screw around today.