What was found
(CAN) DEICE PRESS CAUTION ILLUMINATED IN FLIGHT UPON SELECTING AIRFRAME SELECT MODE TO FAST, RESULTING IN A DIVERTED LANDING. CDS SHOWED NO FAULTS. PERFORMED FUNC TEST OF AIRFRAME PNEUMATIC DEICING SYS IAW Q400 AMM 30-11-00-720-801 TWICE. FIRST TIME FUNC TEST PASS. SECOND FUNC TEST FAILED WHEN FAST MODE SELECTED. OUTER LEFT WING BOOT LIGHT DID NOT ILLUMINATE FOLLOWED BY DEICE PRESS CAUTION, AND LOGGED A ` LOW 1A PRESS` FAULT IN CDS.` R+R #1 DDV IAW Q400 AMM 30-11-16-000/400-801. PERFORMED OPS TEST OF DDV`S IAW Q400 AMM 30-11-16-710-801. OPS TEST PASS, NO FAULTS RETURNED. DDV S/N 1045 TSI: 421 FH, 520 FC (PREVIOUSLY REMOVED SERVICEABLE FROM MSN 4081 WITH 6,698 FH WHEN THE AIRCRAFT WAS PLACED INTO STORAGE). (TC# 20190529020)