As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED. AT BOS, CREW REJECTED TAKEOFF AT 90 KNOTS DUE TO FEEL DIFFERENTIAL WARNING LIGHT ILLUMINATED. AIRCRAFT RETURNED TO GATE. FLIGHT CONTROL SWITCH A FOUND IN OFF POSITION WITH GUARD DOWN COVERING SWITCH. SYSTEM TESTS NORMAL, PER FIM 27-31-00-810-815.