What was found
DURING FLIGHT 3405, CREW REPORTS WHITE AND AMBER PSI LIGHT ON AT 23000 FT. AIR RETURN TO MSP. MAINTENANCE REPLACED CONTROLLER FOR PRECAUTION P/N 304A000-003 (OFF), P/N 3040A000-004, S/N OFF 0032, S/N ON 0007, OPS CHECK AND INSTALL IAW 21-30-02.