While recently installing my first decoders to some Kato F3 units, I found myself scratching my head over what to do with the B unit decoder boards. In the Katos (and likely others), the mechanism has the same engineer seat, etc. as the A units, so the LEDs can be formed to fit rather than snipping them off. I went ahead and did that, so I now have B units with interior lighting. Obviously not useful for much, but I have found it a good way to identify which unit is which (by toggling the headlight) and to verify that the decoder is "on line." What do the rest of you do?
If you are using Digitrax, and you ever want to use transponding, I believe you have to either leave the LED or replace it with a resistor for transponding to work.
Leave it there and turn it off. Once you set it up as a consist with the A units, you'll never have those lights on anyway.
I have snipped them on mine . I did the same on the B end of my E8 , the decoders for the E8 are the same as the P42 that use both lights . I was thinking of using a mini mini LED connected to the terminal of the decoder with a piece of fiber optic to install a light on the right side of the end door near the diaphragm , IM units have it ( simulate and not working ) on their FP7 , I have seen B units being moved by the hostler around engine facility with that light on .