What was found
APU SHUTDOWN CYAN CAS: WAITING TO TAKEOFF, RED APU FIRE CAS MESSAGE ILLUMINATED MONENTARILY FOLLOWED BY CYAN APU SHUTDOWN, APU FAULT, & FIREX BTL 2 LOW MESSAGES. B3 CODES ARE 006348, 008128, 008137. INSPECTED TAIL CONE & FOUND WIRE MESH FALLING APART AND BLOCKING APU EXHAUST. REMOVED LOWER PANEL & FOUND ONLY BLACK RESIDUE FROM APU BACK FIRING DUE TO EXHAUST BLOCKED. CLEANED APU & FOUND NO DAMAGE. PERFORMED OPERATIONAL CHECK & LEAK GOOD. PER AMM REV 29 & MM 49-24-59. REMOVED APU MAG OIL DRAIN AAEMBLY IAW AMM 49-49-13-00-801 STEP 1A-D 1THRU 12 INSTALLED A NEW APU OIL FILTER ELEMENT WITH NEW O-RING IAW 49-91-13-400-801 STEP F. INSTALLED NEW PACKING ON APU OIL FILTER COVER. LEAK CHECKED GOOD IAW MM 49-91-13.