What was found
(CAN) DURING CRUISE, CREW RECEIVED A MASTER CAUTION FOR ENGINE ADP HEAT NR 2 CAUTION LIGHT, AND THE C/B WAS POPPED. AFTER LANDING THE CB WAS RE-SET AND OPS TEST OF THE NR 2 ENGINE AIR INTAKE ANTI-ICING SYS WAS PERFORMED IAW AMM 30-21-00-710-801 WITH NO FAULTS FOUND. THE ACFT WAS RELEASED FOR SERVICE. ON THE NEXT FLIGHT THE ENG ADPT HEAT NR 2 CAUTION LIGHT ILLUMINATED IN FLIGHT AND THE ASSOCIATED C/B POPPED AGAIN. THE ACFT RETURNED TO THE DEPARTING STATION. THE TMU PN 4100S018-07 WAS REPLACED IAW AMM 30-11-01-400-801 AND TESTED SERVICEABLE. AC WAS OPERATED FOR 40 MINUTES WITH ENGINES RUNNING AT FLIGHT IDLE AND ABOVE, WITH NO INDICATION. NO FURTHER DEFECTS ON SUBSEQUENT FLIGHTS.