What was found
WHILE OPERATING THE APU, WITH THE NR 2 GENERATOR PLACARDED INOPERATIVE, THE APU GENERATOR DROPPED OFF LINE ENROUTE FROM PDX TO IAH. THE GENERATOR COULD NOT BE RESTORED. THE CREW ELECTED TO DIVERT TO DFW DUE TO BAD WEATHER AT IAH. THE AIRCRAFT LANDED WITHOUT INCIDENT. MAINTENANCE FOUND THE APU GENERATOR CONTROL UNIT AT FAULT. THE APU GCU WAS REMOVED AND REPLACED. OPERATIONAL CHECKS WERE GOOD.