What was found
PDX - FLIGHT 403QX - THE MASTER WARNING AND CHECK FIRE DETECTION LIGHTS ILLUMINATED IN FLIGHT. THE APU FIRE BOTTLE LOW LIGHT WAS ALSO ILLUMINATED AND STEADY AFTER RUNNING CHECK LIST. THE BOTTLE LOW AND MASTER WARNING ILLUMINATED OFF AND ON FOR A WHILE AND THEN STAYED OFF. UPON LANDING THE APU FIRE WOULD NOT TEST. MAINTENANCE REMOVED AND REPLACED THE APU FIRE CONTROL RELAY AND PERFORMED OPERATIONAL TESTS OF THE APU FIRE DETECTION SYSTEM IAW 8-400 MM TASK 26-13-00-710-801. OPS CHECK GOOD. MTX ALSO CHECKED CONNECTORS ON APU FIRE BOTTLES, APU FADE AND RELAY BOX. ALL CONNECTORS SECURE AND APU FIRE BOTTLE INDICATOR SHOWS GREEN LIGHT. OK FOR SERVICE.