As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RETURNED TO IAD UNDER AMBER ALERT DUE TO LEADING EDGE SLAT FAILED TO RETRACT. *S/D* THE POWER DRIVE UNIT AND M604 ROTARY VARIABLE DIFFERENTIAL TRANSDUCER WAS MISRIGGED. REPLACED ALL PROXIMITY SWITCH ELECTRONIC UNIT CARDS. ADJUSTED M604 ROTARY VARIABLE DIFFERENTIAL TRANSDUCER PER MM-27-51-26. REPLACED OUTBOARD LEADING EDGE SLAT POWER DRIVE UNIT. FINAL TROUBLESHOOTING LED TO REPLACING THE FLAP SLAT ELECTRONIC UNIT AS A PRECAUTION.