As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RECEIVED A SERVICE DOOR CAUTION MESSAGE DURING CLIMBOUT. FOLLOWED QRH AND RETURNED TO LAX. FOUND THE 4 SCREWS HOLDING THE SERVICE DOOR INNER HANDLE TO BE LOOSE. TIGHTENED THE SCREWS AND CHECKED THE GAP FOR THE SERVICE DOOR PROXIMITY SWITCH. ALL CHECKS GOOD AT THIS TIME. SERVICE DOOR INDICATION IS GOOD.