Technical Only reading 50millivolts on the ECU ?

[WitleyPanda]

By product of hooking up Multiscan, noticed that only got 0.050 volt to ECU but that the internal battery 100%.. Main battery over 11v, been sat a forthnight but over 14v when engine running, but still just 45-50 mV on ECU. Notes to check connector.

So question is Where/which one?