As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING FIRE DETECTION TEST #1 FIRE SWITCH LIGHT DOES NOT ILLUMINATE WITH SWITCH IN BOTH POSITIONS. WHEN SWITCH IS MOVED TO B SIDE AN AMBER FAULT LIGHT ILLUMINATES. WHEN SWITCH IS MOVED TO A SIDE THERE IS NO FAULT LIGHT AND FIRE SWITCH #1 ILLUMINATES. GROUNDED - YES. FOUND LH ENGINE FIRE LOOP B HARNESS MW0326 BROKEN CERAMIC TERMINAL. REMOVED AND REPLACED LEFT ENGINE CORE FIRE DETECTION HARNESS REMOVAL REF B737-800 AMM 26-11-02/401.