What was found
LAS - AIR TURN BACK - FLT 837 - DURING CLIMB-OUT CREW RECEIVED NR 1 PACK FAULT WITH THE NR 2 PACK BEING DEFERRED. CREW CYCLED THE NR 1 PACK SWITCH TWICE WITH NO HELP. CREW DECIDED TO RETURN TO LAS. REPORTED THAT ON DOWN WIND, TIRED THE NR 1 PACK AGAIN AND IT WORKED. AIRCRAFT LANDED IN LAS WITHOUT FURTHER EVENT. MAINTENANCE OPS CHECKED THE NR 1 PACK, OPERATIONAL CHECKS WERE GOOD ON THE NR 1 PACK WITH NO FAULTS BEING FOUND. THERE WAS NO PREVIOUS HISTORY ON THIS PACK. MAINTENANCE CLEARED THE DEFERRAL ON THE NR 2 PACK BY REPLACING THE BYPASS VALVE IAW AIRBUS 319 AMM 21-61-53-400-001. OPS CHECK WAS GOOD.