What was found
LOSS OF THRUST ON CLIMB OUT PASSING 5000 FEET, LEFT ENGINE EPR AT 1.44 - EPR INSTANTLY (1 SECOND) WENT TO 1.42, THEN BACKTO CLIMB (1.44) PERFORMED NORMAL REST OF FLIGHT. MAINTENANCE ACCOMPLISHED MULTIPLE ENGINE TESTS. ACCOMPLISHED VIBRATION SURVEY TEST 8 IAW AMM 71-00-00-700-022B. NO ABNORMALITIES OR FAULTS NOTED. ACCOMPLISHED TSM 77-00-00-810-801-A FADEC 1B FAULT CODE N1L AND MESSAGE N1 SENS/HC/EEC1. SWAPPED B CHANNEL SPEED PROBE NR 3 TO SPARE FAN SPEED PROBE NR 2 IAW AMM 77-11-00-860-010-A. ALL CHECKED NORMAL, NO FAULTS. AIRCRAFT WAS GROUNDED.