As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 6578 - IAD-PWM - DURING CRUISE RETURNED TO IAD DUE TO THE LEFT DUCT FAIL LIGHT ILLUMINATING. AIRCRAFT LANDED AT IAD WITH NO FURTHER INCIDENTS. MAINTENANCE FOUND THE LEFT ACM FROZEN. MAINTENANCE UNFROZE THE ACM AND PERFORMED OPS CHECK, OPS CHECK GOOD. AIRCRAFT WAS RELEASED AND APPROVED FOR RETURN TO SERVICE. (M)