a standard cable will not read the airbag ECU. there is only 1 pin in the cable that actually talks to the ECU, so you can only communicate with the ECU on that designated pin, all ECUs have a different pin. the cables by default are set to the engine ECU pin.
I think, but don't quote me that you need to be on pin 3 for the airbag ECU where as the engine ECU is on pin 7, from memory.
Please check or await confirmation that I have the right pins there, I wont be held responsible for any damage done if its wrong as wasn't checked!
the airbag ECU runs a self diagnostic routine on boot every time so provided you get the correct fault code and can fix it the codes should clear themselves.
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