How can i fix warning light to come on when key turned on fiat seicento?

In that case probably just a blown bulb, and there wont be any fault codes stored so no need for diagnostics.
Afraid I can give no advice on how easy or difficult it is to replace the bulb on the Fiat (Fix It Again Tomorrow!).
Best wishes

Fred
August 2011

Thanks very much Fred. As i am the only owner with 17,000 miles the light wasn't disabled and it always passed before,so, it looks like i will have to bring it to someone that can put it right. Thanks again for your time Fred. Nora

Nora
August 2011

I suspect either the bulb has failed, or worse (and sadly more likely) the previous owner has removed the bulb to cover up a fault. Only way to find out is to remove the dash.
Once you have replaced the bulb you will probably find it stays on permanently when you start the car. This indicates there is a fault in the system and the airbag system is dis-abled.
The car will now have to be plugged in to a diagnostics computer which will read the fault code(s), and reset the system. If the light still stays on after the reset you will now have to start looking at wiring and components as guided by the fault codes.
There is no point attempting to fix this without using the diagnostics computer.

fred
August 2011