I have a few 3.5v decoders and used CV8 method, But, cycle track power and about six seconds later the headlight will flash a number of times for a successful reset. Loco runs on address 3.
I use a NCE Power Cab.
I have seen a few times in a forum that some clip a 1k resistor across the rails when using a Digitrax system.
You may be trying to make the ‘wrong’ decoder reset…by using an address the decoder no longer recognizes as itself.
Acquire Address “00” on your DT400 and enter Ops Mode programming. Then do CV8 with the value of “08” and be sure to shut off power to the tracks immediately afterward. Power the rails once more and see if you get a reset.
If that doesn’t work, you need a new decoder or your DCS is not working properly. In fact, it wouldn’t hurt to reset the DCS if it won’t cause you a lot of grief, and then try the reset using Add “00”.
Remove any other engines on the track if you use Selector’s method. Programming Address 00 on the main almost always works but it will reprogram everything on the tracks.