Is it an O2 sensor ?

About 8" from the exhaust port at the heads?  O2 Sensor.

Maybe a pic......

Come on Paul what is the fun on that ;)

It’s ya flapper valve

If you have a flash tuner turn it off in the cal and unplug it, you can then take all the actuator stuff off, if you can't turn it off get a dealer to do it for you. It's just a valve to limit gas flow.

Grease Monkey is on the money. It acts like a butterfly valve and is a twofold purpose. Supposed to increase TQ at lower rpm by remaining closed then opening up for better flow at higher rpm. The second was to limit exhaust noise for epa at lower /idle speeds. As Grease Monkey said you can deactivate it with your tuner

Fugg me this question 13 years on !

This is the "Active Exhaust" in Harley speak , it is NOT on US "Domestic" bikes only on some market 5HD vin numbers including EUROPE

We in Australia get the Euro spec bikes 100% ( bar the LHD headlights !). Have done since October 2006

I said the 'zact same thing to Frasers HD in Perth in October 2006. "What the fuggzthat ?" Re my new 2007 FXSTC just arrived in Oz

AND THEY DID NOT KNOW I KID YOU NOT !

So I found out and told 'em you betcha. I Worked out you could "fool" it and not trigger a fault code by just screwing or riveting the flapper in the open position.

So I tells Frasers and they started doing it at PDI on many bikes. I did many bikes for mates , and 13 years on they are still running allright, flapper screwed open.

I have now worked on many "Grey imports" both from the USA and Japan and they do not have this "Active exhaust' system. ( we also get "active air" they don't !)

It is not to increase torque , it is purely for Euro spec "drive-by" noise testing while you are in metro areas like Paris for example.

They strangle the bikes to buggery and always did.

If you change out the stock headers , as in fitting a V&H system for example you just unbolt and unplug all that bullshit along with the stock exhaust.

If you have a "race tuner" for the bike then there are calibrations or "'MAPS" that allow you to switch off or more accurately "disable" active exhaust and air.

You may then have to over flash the bike again with a calibration for your model/cams etc etc. Not all calibrations allow toggling OFF and ON