As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
NEITHER AUTOPILOT WILL NOT ENGAGE(DISCOVERED AUTOPILOT STAB TRIM SWITCH OFF! - OPERATED NORMALLY AFTER SWITCHED ON). GROUNDED - YES. BOTH AUTOPILOT WORK NORMAL, NO FAULT FOUND DURING BITE TESTBOTH CHANNEL A AND CHANNEL B LRU TEST PASSED. REF B737-900 FIM 22-11 TASK 856PITLO CONFIRMED THE AUTOPILOTS WORK NORMAL, FORGOT AUTOPILOT SW OPERATE(CUTOUT).