As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB AFTER ENGAGING THE AUTOPILOT, THE CREW NOTICED THE A/P TRIM FAIL LIGHT ILLUMINATED ON THE MASTER CAUTION PANEL, THEN NOTICED THE AUTOPILOT ADVISORY THAT THE MACH TRIM HAD FAILED. THE CREW RETURNED TO HOME BASE (KHOU). DURING TROUBLESHOOTING FOUND CONTACTOR 72C, MS 24187-D1 HAD AN OPEN CONTACT. THIS RELAY IS THE AUTOPILOT TRIM DOWN RELAY.