The canbus shield works only on newer vehicles that support canbus.
Take a look at the STN1110 website.
STN1110 - Multiprotocol OBD Interpreter IC Chip - Low-Cost, High-Performance | OBD Solutions
It has optimized algorithm to discover automatically the protocol of OBD2 system..
Support for all legislated OBD II protocols:
ISO 15765-4 (CAN)
ISO 14230-4 (Keyword Protocol 2000)
ISO 9141-2 (Asian, European, Chrysler vehicles)
SAE J1850 VPW (GM vehicles)
SAE J1850 PWM (Ford vehicles)
Support for non-legislated OBD protocols:
ISO 15765
ISO 11898 (raw CAN)
Support for SAE J1939 OBD protocol
You have to worry for less things on your project because, like ELM327, STN1110 does the most things for you.