What was found
"PFD 1 MON FAIL" MESSAGE ON ED. C/A FD HAS MULTIPLE FAILURES OF ALL INSTRUMENTS (RED X'S, ATT FAIL, IAS FAIL, ALT FAIL, HDG FAIL). RETURNED TO FIELD, LANDING UNEVENTFUL. INSPECTED GROUNDS FOR NR1 PFD AND NR1 ICP. ALL GROUNDS WERE SECURE. REPLACED NR1 PFD IAW TC 6316104, ALL CHECKS GOOD. ACFT CONTINUES CAT III, PLACARD READS CAT III. RETURNED TO SERVICE.