What was found
EN ROUTE CVG, LEFT AC BUS OFF, DC BUS OFF, EMER AC BUS OFF, AC CROSSTIE LOCKOUT LIGHTS ILLUMINATED, REF POM PROCEDURE: EMER POWER ON, RECOVERED EMER AC BUS. DIVERTED KIAH, EMERGENCY DECLARED. REMOVED AND REPLACED THE APU GCU WHICH HAD A DIST SYS FAULT. RAN APU NR 1 AND 2 ENG''S PER PFHB AND TRANSFERRED POWER SOURCES, ALL INDICATIONS NORMAL.