I have a DTS but the nav system is essentialy identical. I know you all have had the 1791 code for a while. This one allows you to play the DVD while in drive. I have another code, and I didn't think anyone would be too interested in it, but since the 1791 code is so popular I figured I'd share it. This provides access to the general diagnostic features of the nav system. You can see satellite signal info, all the nav systems inputs, do tests of the mic and speakers, check software version, etc etc. There are several calibrations that you can make, and I suggest you avoid these unless you know what you're doing. I hope I'm not opening pandora's box here: 1971 (yes, very similar to the other code. Have fun.