As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BOTH FIRE LOOP A AND B STATUS MESSAGES PRESENT AFTER DET FIRE/ OUT 1 EICAS MESSAGE. THIS RESULTED IN AND AIR TURN BACK. C/W FIM 26-11-00-810-818 AND AMM 26-11-00-730-801, FOUND FIRE DETECTOR LOOP A AND LOOP B HAVE HIGH RESISTANCE. REMOVED AND REPLACED FIRE DETECTOR TS883 PER AMM 26-11-02/401. RESISTANCE CAME BACK INTO RANGE. OPERATIONS CHECK GOOD.