As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
"CHECK FIRE DETECT" WARNING LIGHT ILLUMINATED, ASSOCIATED WITH THE APU "BTL LOW" ADVISORY LIGHT. REPLACED APU FIRE BOTTLE PER MTC 6262301, 6262302 AND HORIZON AIR EA 6-26-17, PERFORMED OPERATION TEST OF APU FIRE DETECTION SYSTEM PER Q400 AMM TASK 26-13-00-710-801, APU CHECKED GOOD. ACRFT RETURNED TO SERVICE.