Try this: http://www.totalcardiagnostics.com/s...d-obd-software

I've spent a considerable amount of time troubleshooting this. If you're using an APU1, what I've found out is TPRT doesn't let you change the COM port when APU1 is selected. With that said, set APU1 to pass thru, open device manager, plug in USB and observe which COM port it comes up with. Launch TPRT, Tools>Preferences, if not the same change COM port to the same number as in the device manager and "Test for valid interface..." Switch from pass thru to APU1, but you'll no longer be able to "Test for valid interface..." however it should detect hardware when "initialized." Also on XP, it required a reboot. Can't remember which Win 7/10, had to close and relaunch TPRT. Next on the agenda...Surface Pro w/ Win 10. YMMV