have a look at ELM 323 and its varients, its a pic chip that will interface the OBDII of the VW to RS232 with simple 'AT' commands,, fault codes can be read/cleared but sensor data can be captured and logged if you interface it with a PIC/AVR etc and say a eeprom...you will need to know what version of the OBD your vw speaks, the elm chip will not work with the earliest versions its ok with KWP2000 and later...loads of circuit diagrams on the elm website and the net
http://www.elmelectronics.com/DSheets/ELM323DS.pdfif you just want to monitor sensor data and/or clear fault codes try vag-com..hard to beat
