Bought a set of Vance and Hines Short Shot pipes through Lofty for a Dyna a while back, and the fellas just gone to try and fit them. He found that there was a 3rd sensor in the pipes he's got, which are standard with S&S Slip ons. What is it for, and what does he do with it? Is it one that can be left off? Or does it need to be added?

I'm guessing you are talking about the Exhaust Flapper valve ? Try a search.
If so, just remove the cable from the servo, but leave the servo plugged in.

I was just thinking if the OP doesn't know about the flapper valve, he wont have any knowledge of TTS/Sert etc'
But that's a good point, it would be wise to re-map with the new pipes anyway.

 

All fixed, he rand PS and they told him what to do.

thanks for the advice, always handy