What was found
CLIMBING THROUGH FL400, AN ODOR WAS NOTICED BY CREW, FOLLOWED BY VISUAL SMOKE IN BOTH THE CABIN AND COCKPIT. SHORTLY AFTER, TWO AMBER CAS MESSAGES FOR ACM OVERTEMP AND EMERGENCY PRESSURIZATION ILLUMINATED. EMERGENCY DESCENT WAS ACCOMPLISHED ALONG WITH COMPLIANCE WITH THE CHECKLIST. DIVERTED. REMOVED AND REPLACED THE RT MASS FLOW SHUTOFF VALVE DUE TO BEING FAILED IN THE CLOSED POSITION. OPS AND LEAK CHECKED GOOD IAW AMM 21-10-00, VERIFIED OPERATIONAL OF ECU /ACM WITH ENGINES AND APU RUNNING WITH NO SMOKE OR ACM OVERHEAT MESSAGES.