What was found
AIRCRAFT WAS GROUNDED: PILOT REPORTS AIRSPEED DISAGREE / ALT DISAGREE CAUTIONS, AIRSPEED LOW WARNING IN FLIGHT FL 280, EEC (S) WENT TO ALTERNATE MODE. EVENTUALLY FO SIDE RETURNED TO NORMAL, FOLLOWED BY CA SIDE RETURNING TO NORMAL APPROX. 5 MIN LATER. ALL ENGINE HEATS / PITOT HEATS TO PROPER POSITIONS. NO OTHER WARNINGS & CAUTIONS. MAINTENANCE FOUND THE L/H PITOT AT FAULT, REPLACED IAW AMM 34-11-01 LEAK AND OPS TEST OK.