What was found
DURING TAXI OUT, LEADING EDGE ASYM EICAS MESSAGE,AFTER EXICUTING AFTER START CHECK LIST . AIRCRAFT RETURNED TO BLOCKS (BTB). MTX GROUNDED A/C . C/W OPS CHECK OF THE LE SLATS IAW AMM 27-51-00. FLAPS FAIL LE SLAT ASYM MSG APPEARS . FSEU SHOWING MSG 27-81021. REF FIM FSEU BITE 27-51-FT809 OSEU TARGET CHECK OF S278 SWITCH SHOWS SENSOR READING NEAR WHEN IT SHOULD BE FAR , REMOVED AND REPLACED PROX SWITCH S278 IAW SWPM 20-10-13,20-10-14. OPS CHK FAIL .FURTHER T/S REQ. SEE LP 39227-19/2 PERFORMED CHECK OF THE CIRCUIT BETWEEN THE #3 SLAT OUTBD PROX SENSOR AND THE PSEU CONNECTOR D2166A PINS E10, F10. MH READING GOOD. REMOVED AND REPLACED PSEU IAW AMM 32-09-04, OPS CHKS OK. NO EICAS MSGS S278 PROX SWITCH INDICATE ACCORDINGLY. ALSO REF. WDM 27-88-11.LP 39227-19/2. A/C RETURNED TO SERVICE