2005 Citroen C3 Engine auto-diagnosis light

Hi hope you can help car shuddered a couple of times then audable beep and the engine auto-diagnosis light came on. My garage thought it was just miss firing and it was just a spark plug problem but doesn't seem the case and they are unsure at the moment what the fault is are you able to shed any light on what the problem could be. The car has been running without any issues at all up until now.
Thanks