Help us help you... Little more about the problem please.
What year Mustang? T45 or TR3650 or 4R70W trans
? What prompted you to install a new VSS? Is the speedo reading correctly? Have you viewed live data tables to see what the pcm "thinks" vehicle speed is, and more or less confirmed that to be true or false? Any drivebility issues (would likely be possible if auto trans
)? And what dtc exactly are you getting?