As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB, THE LEFT MAIN LANDING GEAR DOOR OPEN LIGHT ILLUMINATED AFTER LANDING GEAR RETRACTION. FOLLOWED COCKPIT OPERATING MANUAL PROCEDURES AND LIGHT REMAINED ILLUMINATED. AIRCRAFT RETURNED TO DTW AND LANDED WITHOUT INCIDENT. REPLACED THE LEFT MAIN LANDING GEAR DOOR MICRO SWITCH AND OPERATIONAL CHECK GOOD.