What was found
DURING CLIMB OUT, "PWS" FLAG ILLUMINATED ON ND'S THEN CLEARED AFTER APPROXIMATELY 3 MINS. GROUNDED - YES. INITIAL EVALUATION, NO MSG ON ND AT POWER UP AFTER ALIGN AND WXR SELECTED. PERFORMED WXR BITE IAW B737-800 FIM 34-43 TASK 801, NO FAULTS FOUND. PERFORMED EGPWS BITE IAW FIM 34-46 TASK 801, NO FAULTS CURRENT OR IN HISTORY. REF FIM 34-43 TASK 815, F(1)E. "NO FURTHER ACTION REQUIRED." 2ND DEFECT IN 10 DAYS. AT THIS TIME, R/R OF DRIVE UNIT NOT REQUIRED. AS PRECAUTIONARY, LIGHTLY CLEANED/DUSTED WITH CANNED AIR AROUND BLOWER UNIT ASSY. BLOWER UNIT TEST SW, CHECKS GOOD. NO DEFECTS NOTED. IF PROBLEM PERSISTS, R/R OF DRIVE UNIT MAY BE REQUIRED PER THE FIM.