Hi Ross and welcome on board !
a correct OBD reader would tell how various parameters were when the MIL popped-up, if the light is still ON, the error is still present (or at least stored in the ECU memory). Would be nice to have the readings before attempting anything... Got some ££ to spend ? Get an ELM interface and the
MES licensed software for about 80€ total; well spent money if you own a Fiat ;-)
BTW: no clue about this 14H/321 code; could be coming from their diagnostic machine itself aswell !!
Regards, Bernie