As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CLIMBING THROUGH FL210 WE NOTICED A RED UNSAFE LIGHT IN THE LEFT MAIN LANDING GEAR WITH THE HANDLE IN THE UP POSITION. COMPLIED WITH THE QRH PROCEEDURE. RED LIGHT REMAINED. MTC REQUESTED A RETURN TO ATL. REPLACED LT MAIN GEAR UP AND LATCH PROXIMITY SENSOR D1-26 PER AMM 32-61. ACCOMPLISHED GEAR SWING PER 32-61-02, ALL INDICATION NORMAL. (LP#3795176)