Any OBD diagnostic software will be able to give them the fault code the car is generated. Some codes are generic across all brands and mean the same thing no matter what and some are manufacturer specific.
If they have good Alfa diagnostic software it should be able to give them a relatively accurate description of what has happened.
The trick you will face is whether or not the warrant will cover it, some of them are real micky mouse warranties. It will be a sensor that reports the fault, it has to be, so the question is whether something has actually gone wrong or if a sensor has worn out. If sensor has worn out then it could be classed as wear and tear and not covered.
My guess would probably be a lambda sensor about to retire!
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso