What was found
WXR RADAR SHOWING FALSE RETURNS IN AUTO MODE WITH WX-T SELECTED IN CLEAR AIR AT CRUISE FL340 WITH NO VISIBLE WEATHER PAINTS 3 CEUS APPROX 1/2 INCH IN DIAMETER ON 80 AND 160 NM RANGE, AT 2:00- 2:30 POSITION ON ND. CEUS DO NOT APPEAR IN MANUAL MODE. LASTED 30 MINUTES,THEN DISAPPEARED. GROUNDED - YES. PERFORMED WEATHER RADAR SYSTEM BITE PER B737-900 FIM 34-43-00-810-801 AND WEATHER RADAR SYSTEM OPS TEST PER B737-900 AMM 34-43-00-710-802-001. NO FAULTS NOTED & ALL TESTS PASS. NO HISTORY, CLEARS MEL 34-15D.