We suggest that when trying to program an FL4 on the programming track you connect a 100 ohm resistor between one of the function wires and the blue wire and that you also have no light bulbs or LEDs connected to any of the function wires. To achieve this, it is probably best if you program your FL4s using alligator leads before you install it into the locomotive or car and before you have any light bulbs hooked up to it. This protects you from having to unsolder the lights from the decoder in order to program it. Of course this is only for programming on the programming track, the decoder will program fine on the main track no matter what, so the only thing that you have to program before you install it is the address, everything else can be programmed on the main in ops mode. If you would like a more detailed reason for why this is continue reading. When a command station reads back CVs on the programming track it first sends out a packet of information that the decoder must respond to so that the command station knows that there is a decoder actually there. According to NMRA standard the response from the decoder must be a minimum of 60mA, so the way decoders send that response is they send a short burst of electricity to the motor, the command station senses the rise of current on the track and then reads back the CV. The problem, however, is that the FL4 does not have a motor circuit so we have to use the function leads to raise the current on the programming track and most bulbs or LEDs draw anywhere from 10-30 mA of current, which is only 1/6 to 1/2 of the current needed. This, of course, is not enough current for the command station to recognize, so you cannot read back CVs on the programming track using bulbs or LEDs. As a way to get around this, we suggest attaching a 100 ohm resister between a function lead and the blue wire, which is the positive common. At a track voltage of 12 volts a 100 ohm resistor would be drawing 120 mA of current, which is two times the amount necessary for a command station to recognize a decoder on the track. The problem, however, is that command stations also have a maximum current allowed on the programming track, and that maximum current is not regulated by NMRA standards. So if you have a 100 ohm resistor connected to a function lead and you also have bulbs or LEDs on other function leads you may then be drawing too much current and then you will also not be able to read back CVs. This is why we suggest that you program on the programming track with only a 100 ohm resistor connected to one function lead, and nothing else hooked up to the decoder.