B757 captain reported an APU malfunction upon start up at the gate.

Attributes
ACN	1579645
Time
Date	201809
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Light	Daylight
Aircraft 1
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Parked
Flight Plan	IFR
Component
Aircraft Component	APU
Person 1
Function	Captain
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Experience	Flight Crew Total 16649 Flight Crew Type 13269
Events
Anomaly	Aircraft Equipment Problem Less Severe Flight Deck / Cabin / Aircraft Event Other / Unknown

Narrative:

At [the gate] performing preflight duties for [our flight]; APU was started and external power was deselected. First officer (first officer) went outside to perform walk around and observed smoke from APU tailpipe and was told by ramp personnel that smoke was coming from APU. First officer returned to cockpit and informed me. There was no APU fire indication. I went to observe and was met by mx personnel at jetway. The smoke had subsided but was told by mx that there had been excessive smoke. I returned to cockpit with mx and APU had auto shutdown in last minute or so. I made PA and told pax to deplane the aircraft through jetway to terminal area. There was no smoke in cabin. Mx positioned at APU tailpipe with fire extinguisher and informed [flight department]. Mx informed me extinguishing agent was used in tailpipe. Mx made logbook entry of APU smoke and auto shutdown. I discussed aircraft with dispatch and refused aircraft.

Original NASA ASRS Text

Title: B757 Captain reported an APU malfunction upon start up at the gate.

Narrative: At [the gate] performing preflight duties for [our flight]; APU was started and external power was deselected. FO (First Officer) went outside to perform walk around and observed smoke from APU tailpipe and was told by ramp personnel that smoke was coming from APU. FO returned to cockpit and informed me. There was no APU fire indication. I went to observe and was met by MX personnel at jetway. The smoke had subsided but was told by MX that there had been excessive smoke. I returned to cockpit with MX and APU had Auto Shutdown in last minute or so. I made PA and told pax to deplane the aircraft through jetway to Terminal area. There was no smoke in cabin. MX positioned at APU tailpipe with fire extinguisher and informed [Flight Department]. MX informed me extinguishing agent was used in tailpipe. MX made logbook entry of APU SMOKE and Auto Shutdown. I discussed aircraft with Dispatch and refused aircraft.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.