What was found
OUT OF 15,000 FEET AND CLIMBING, LEFT NP INDICATION BOTH ANALOG AND DIGITAL BECAME ERRATIC THEN DROPPED TO ZERO. ON DESCENT, INDICATIONS RETURNED TO NORMAL THEN BECAME ERRATIC AGAIN THROUGH 8,000 FT. RETURNED TO LAX. REPLACED NR 1 NP INDICATOR AND RAN LEFT ENGINE FOR 30 MINUTES AT DIFFERENT SPEED SETTINGS, NP INDICATIONS OPS CHECKS GOOD.