What was found
NO PAX ONBOARD. WHILE EN-ROUTE, RECEIVED NUMEROUS FAIL & FAULT INDICATIONS, IE YAW DAMP, PITOT HEAT, FLAPS, DCU, PROX SYS. DIVERTED BACK & AFTER ENG SHUTDOWN ON GND, PLUS ABOUT 15 MINUTES, HAD SMOKE & FUMES IN COCKPIT. SHUTDOWN ALL SYS & EXITED ACFT. NO CURRENT FAULT AFTER POWER UP & REVIEWED FAULT HISTORY. ABLE TO DUPLICATE FAULTS BY PULLING DCU A BREAKER. R & R DCU IAW AMM 31-41-01-000-802. OPERATIONS TESTED GOOD IAW 31-41-00-710-801. NOTE NO RECURRING ISSUE TO DATE.