As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LT PACK LIGHT ILLUMINATED ON GROUND WITH APU SUPPLYING AIR AFTER ABOUT 3 MINUTES. GROUNDED-YES. REMOVED LT PRIMARY HEAT EXCHANGER AND PLENUM/DIFFUSER. REPLACED DAMAGED DIFFUSER. REINSTALLED HEAT EXCHANGER AND PLENUM DIFFUSER. OPERATIONS CHECK GOOD, REF B737-900 AMM 21-51-03-400-804-002. RAN PACK FOR 30 MINUTES ON GROUND, OPERATIONS NORMAL. TEMP CONTROLLER BITE SHOWS NO FAULTS.