What was found
T/O, 1 MIN NR 1 ENG FUEL FILTER CLOG LT ILLUMINATED FOLLOWED QRH NO RELIEF RTB LAND WT 271.5K LBS. RELEASED FOR SERVICE IAW MEL: 73-36-03-01. ONCE EACH FLIGHT DAY." NR 1 ENG FUEL FILTER INSPECTED & NO CONTAMINATION FOUND. R & R FUEL FILTER IAW AMM 73-12-03-4. CARRIED OUT LEAK CHECK, NIL LEAK IAW JRH. ACFT OK FOR SERVICE. REPLACED DIFF PRESS SWITCH, PIN C SHORTING. WHEN INSTALLED THE PLUG (DS298) WITH NORNAL POW ER,THE LIGHT"FILTER" EXTINGUISHED,BUT INSTALLED THE PLUG A LITTLE TIGHT, THE LIGHT "FILTER" ILLUMINATED. CHECKED PLUG (DS298) GOOD, SUSPECTED THE PIN C OF SENSOR GROUNDED WHEN PLUG (DS298) INSTALLED TIGHTLY. R&R NR 1 ENG FUEL FILTER DIFFERENTIAL PRESSURE SWITCH AS PER AMM 73-34-11-4. OPS CHECKED NORMAL.