Please read this ENTIRE article
1. Remember that you can only READ CVs when your DCC system is in Program Track Mode / Service Mode. Note the use of the word mode. This does not pertain to physical connections. Example: The Power Cab has a single two wire track connection. This means program track mode will make changes to ALL locomotives on the track connected to those two wires. The Power Pro, Ph-Pro, Powerhouse and CS02, all have dedicated programming track outputs labelled as such on the front of the box for this specific purpose. The SB5 does NOT have a program track MODE or a programming track output.
2. When trying to read a decoder and you get a manufacturer ID of "255" and / or multiple readings of 255 from the decoder for other CVs too, that is an error message. A message of "Cannot Read CV" is also an error message. Translation: The system asked the decoder a question but did not get a response back from the decoder. Check all your connections and track. Try a different decoder or locomotive for comparison.
3. When trying to read CVs of a SOUND decoder on an older Power Pro / Ph-Pro system box you might need a programming track booster like the Soundtraxx PTB-100. Do not use a DCC Specialties "PowerPax"! Do not connect any type of programming booster to the Power Cab.
Some decoders cannot be read back no matter what. Soundtraxx Tsunami 2, BLI , MTH, and MRC are almost impossible to read correctly. Use programming on the main (POM) Programming on the Main / OPS mode programming instead. You will not be able to READ a CV, but you will be able to change it. This method is under full track power which some decoders seem to like better. The downside is that you cannot READ a CV using POM.
You can also try DIFFERENT TYPES of CV readback methods. Not all decoders support Direct Mode Programming. They prefer PAGED Mode Programming instead.
Your NCE system manual has detailed instructions for how to access and use the different type of service mode programming / program track programming methods
Cannot read CV means the decoder did not respond to attempts to read this parameter value in the decoder. There are several things that can cause a failure to read: a miswired decoder, broken or missing track pickup wires, the motor is not connected to the decoder, defective decoder, wires to the program track are disconnected, etc. All decoders require that a motor (or similar ‘load’) be connected to the decoder for proper reading on the program track. Decoders that draw too much power from the program track can also cause this message. Most sound decoders draw large amounts of current.
The Power Pro can supply the NMRA limit of ¼ Amp of current for initial ‘charging’ of these decoders and 160mA of continuous current for normal program track operations. If the decoder exceeds these capabilities you will get the error message. Press ENTER to continue. You will probably still be able to program by ignoring the error. Press ENTER, then proceed in the usual manner.
The Power Cab has an extremely high success rate when it comes to reading CV values in program track mode. The most common reasons (in order) you cannot read a CV are:
1. dirty track / dirty wheels / off track
2. broken or loose wires in the loco / installation
3. defective or blown decoder
4. the particular manufacturer that does not support readback
Are you in program track mode or programming on the main? Remember that you cannot read back ANY CV’s on the main. You can only read back in program track mode.
Is the track perfect AND the wheels are 1000% clean?
Just because you cannot READ the CV does not mean you cannot WRITE the CV. It’s called blind push programming similar to programming on the main. This is a fairly common issue and not always an indication of hardware problems. The fact is some decoders will not read back at all but will program just fine. You may get an option to "program anyway". Try that too.
Each time you read or write information to the loco it should move a small tiny bit to visually confirm it received your input and is doing something. If you can't tell it is moving when reading or writing visually, you can lightly rest a finger on the loco and feel for movement.
if you can't get a response of any kind, read a CV, or get any movement when programming, the decoder installation needs to be checked. It could be a faulty decoder or you may not have a decoder installed correctly.
In program track mode press 1 for standard, if you get “cannot read CV”, press enter again, it will try a different read method, it also has a third one too. After that it will dump you into a manual push mode for blind programming. The other menu option is to press 2 for CV, then you can enter the CV and the value directly.
Some decoders will not react to the changes you make until after they are “rebooted” In some decoders the value of the CV is read ONLY at startup. Reboot your system then verify the changes.
If you have an older NCE 5amp system and use sound decoders, a programming track booster might be of some benefit. The Soundtraxx PTB-100 is one alternative for this: http://www.soundtraxx.com/access/ptb100.php. Not for use with the Power Cab!
Note: The Power-Pax programming track booster was tested by NCE but did not work well at all.
if you have a PH-Pro and are comfortable with modifying the command station you might want to try this: https://ncedcc.zendesk.com/hc/en-us/articles/203279359-Improving-the-Programming-Track-on-the-Power-Pro