What was found
LEFT PACK LIGHT ILLUMINATED UNSOLICITED WITH A DUCT PRESSURE DIFFERENTIAL OF 25 PSI. 15 PSI ON LEFT AND 40 PSI ON RIGHT. PACK LIGHT EXTINGUISHED ON MASTER CAUTION RESET. GROUNDED-YES. INITIAL T/S, RECALL TEST FOUND NR 1 PACK LIGHT ILLUMINATED, PZTC NR 1 FAULTED RAM AIR ACT, RESET FAULT REF B737-800 AMM 21-51-00-700-803. OPERATIONS BOTH LT & RT PACK FOR 30 MINUTES, NO FAULT PRESENT & NO FAULT ON RECALL TEST, AMM 21-00-00. UNABLE TO DUPLICATE DISCREPANCY. DUE TO PAST MX HISTORY, REPLACED PZTC NR 1 FOR PRECAUTION. INSTALLATION TEST GOOD AMM 21-61-20/401.