Cable itself. VWTool was the most useful software when it did work. Even with the first cable it was tempermental. The first cable burnt out and died. After receiving the second cable and not being able to connect to my ECU again I tried a second ECU and then a friend's car and second laptop. Nothing worked with the included software, torrented software or VWTool though to cable itself seemed to at least part-work because Windows would detect the USB device and the LED would flicker on it when attempting comms to the ECU. My friend then used another ECU diagnostics device on my ECU (a code reader) which worked fine. I believe Hayesey uses a serial cable rather than USB and has more sucess, though for what information it can actually give even when it does work, it's not really worth the cost or hassel. As Hayesey says somewhere in another post - the best diagnostic tool for the engine management on these cars is a multimeter. I second that advice.