2004 diesel is EOBD .. Which is based on OBD2. Petrol cars are EOBD after 2001. Before the manufacturer could use anything ... so Alfa/Fiat used a hybrid VAG/COM system.I have 2004 jtdm which is definately odb2 compatible. Cheap scanner correctly reads codes, gives freeze frame data and resets codes. I only use fiat FiatECUscan and cable for air bag and potentially abs. Blue tooth plugs also work and i have briefly played with an android app called torque which was able to communicate with my car very successfully. In my experience the 156 is not too different from most other diesels of this era.