As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER DEPARTING EWR AND CLIMBING THROUGH 16000 FT MAIN CARGO DOOR "B" LIGHT ILLUMINATED. COMPLIED WITH QRH AND RETURNED TO DEPARTURE. RELEASED FOR SERVICE IAW MEL: 52-15-01. REMOVED AND REPLACED CARGO DOOR WARNING SYS "B". RELAY R9107. OPS CHECKS NORMAL IAW DOOR INSTALLATION MANUAL. OK FOR SERVICE.