Hi Joe,
What version of DCC-EX are you running?
You can Send a lower case?? s?? status which will display the version in the traffic monitor screen.
The 303 error is saying you're attempting to Read cv's on the Main line track from the A leads. The Main track can only Write to cv's not read them.
Move the engine onto a Programming Track from the wires on the B leads, and it will read the engine decoder.
If you still need additional information, please download this DCC-EX 4.1 & JMRI DecoderPro 5.2 Getting Started Guide.pdf
Hope this helps.
Regards,
Kevin
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