hi joe still think you have a loss of supply and thet the power up lead to the diagnostic plug is back feeding normally with those you can retreve the codes manualy by bridging 2 terminals withing the diag socket would need to confirm pins from auto data. however having used the launch for the stuff the snappy dosent like i have had ocasions when i have had a back feed through the diag power up lead.
The car was running when the belt snapped there for you need to go back and retrace your job step by step and find thet lead you left off or the one thats broken somewhre 9/10 when i go to look at jobs like yours its something incorrectly positioned or not connected.
hope this helps
some times fresh eyes help