What was found
DURING CRUISE AT FL 240 MASTER WARNING ILLUMINATED INTERMITTENTLLY APPROX 3 SEPARATE TIME WITH ASSOCIATED CHECK FIRE DET AAND APU BTL LOW LIGHT ALSO ILLUMINATING. THE APU FAULT LIGHT DID NOT ILLUMINATE. NO FIRE EVIDENCE. REMOVED FROM SERVICE. INSPECTED BOTTLE AND FOUND TO BE AT NORMAL RANGE COMPLIED WITH FIM TASK 26-23-00-810-802 PARA 4 A,1. REMOVED AND REPLACED APU FIRE EXTINGUISHER BOTTLE. REF Q400 AMM 26-23-01-000-801 26-23-01-400-801 AND COC HS# 26-0015 USED THE SAME CARTRIDGE. OPS CK. GOOD NO CHECK FIRE DET. AND APU BTL LOW LIGHT AT THIS TIME. RETURNED TO SERVICE.