As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU COLD SOAK START ATTEMPT UNSUCCESSFUL TWICE. ATTEMPTED APU COLD SOAK START AFTER TWO HOURS AT FL 410. APU SHUT DOWN TWICE AFTER REACHING APPROXIMATELY 750 EGT. FAULT LIGHT ILLUMINATED. REMOVED AND REPLACED THE APU IGNITER PLUG, OPS CHECKED OK IAW AMM 49-41-02.