As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DEN - ON PUSHBACK FOR FLIGHT 4436 RECEIVED A APU FAULT STATUS MESSAGE. RETURNED TO THE GATE. MAINTENANCE INTERROGATED THE MDC AND FOUND A UNDERSPEED MESSAGE POSTED. UPON INVESTIGATION FOUND POWER/FUEL BUTTON DESELECTED AND START/STOP BUTTON SELECTED. DESELECTED START/STOP BUTTON. FAULT MESSAGES CLEARED. OPERATIONAL CHECK OF THE APU IS GOOD. AIRCRAFT RETURNED TO SERVICE. NO PARTS REPLACED.