Thanks SO much for your help. Went back and did what you suggested..everything's working again.
Not sure exactly what you did, but if the PowerCab was asking you for a short address and you put in 001 or 003, that was your mistake. With NCE short addresses are 1 through 127 (the same range as consist addresses). If you put in 001 or 003, those are considered long addresses.
I suggest that you leave all your short addresses as 3. Then select some appropriate long address, and make that the active address.
So put the 1st loco back on the programming track and follow the programming procedure from the start. Set the short address as 3. When it asks you if you want to make that the active address say no. Then set the long address to what you want. If you want to confuse yourself, you can set that address to 003. Then make the long address active.
When you want to run the loco with the long address active, select loco and key in 003 for the address. You should see 3 or 03 on the display with an * in front of it. That means it is a long address.
Do not enter leading zeros for the address -- the NCE system thinks these are long addresses and sets CV 29 to the wrong value.
Reset the decoders to factory default values and start over.
Also, you cannot use Address 1 -- that is reserved for the cab's use.
I installed a DH123D in an old Tyco "plymouth" switcher. After testing the install (headlight worked, forward and reverse worked), I tried to reprogram the short address with my NCE Power Cab system. After following the steps in the reference manual for the system to program on the programming track, I can't command the loco at all with the new address (I put in 001 for a test address). I put another loco on the track with the same decoder which -hasn't- been programmed, and can't control that one at address 003.
So, what rookie programming mistake did I make, and how can I correct it?