What was found
EN ROUTE TO STC W INBOARD LIGHT ILLUMINATED ON IT'S OWN FOLLOWED QRH LIGHT STAYED ON. AIR RETURN TO MSP. MAINTENANCE OHM CHECKED DUCT MATS AND THERMISTORS, OHM CHECKS GOOD. REMOVED AND REPLACED DEICE MODULE, PERFORMED 20 MIN OPS CHECKS, OPS CHECKS GOOD P/N 44730-501, S/N OFF A00072, S/N ON A00581.