Ask client if both went at same time, if so check terminal box on base of hob, wiring may have not been secured tightly, links not fitted properly etc.
Thanks lee8, found burnt section on faulty pcb, connected mains onto right two rings and they work great, contacted customer to give her the quotation to replace the left hand pcb but she wasn’t prepared to spend that much for the repair, thinking of putting the good parts on ebay.