As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN)APU FIRE WARNING ON ON GROUND JUST PRIOR TO TAKEOFF. APU HAD BEEN SHUT DOWN FOR 5 MINUTES. APU FIRE HANDLE PULLED STILL INDICATING FIRE FOR SOME MINUTES. VISUAL CHECK INDICATED NO EVIDENCE OF FIRE. FO UND EXHAUST FIRE DETECTOR PLUG CONTAMINATED BY CARBON AND FLUIDS. CLEANED.