What was found
PDX FLIGHT NR 2335 - APU STARTED NORMALLY AND RAN WITH GENERATOR SUPPLYING POWER, BUT WHEN BLEED AIR WAS SELECTED, APU AUTO SHUTDOWN AND FAIL MESSAGE APPEARED ON APU CONTROL PANEL. RAN EA/CHECKLIST AND RESTORED APU STARTED NORMALLY. MAINTENANCE RAN APU WITH GENERATOR AND AIR. NO APU SHUTDOWN. ALL INDICATIONS NORMAL. PERFORMED FUNCTIONAL CHECK IAW Q400 AMM TASK 49-00-00-720-801. OPS CHECK NORMAL. NO PARTS REPLACED. AIRCRAFT RETURNED TO SERVICE.