As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 191 JFK-LAS, AIR RETURN TO JFK DUE TO AT ALTITUDE WITH NR 2 PACK FLOW SELECTED HIGH (DUE PACK 1 MEL) THE PACK FLOW OSCILLATED BETWEEN FULL LOW AND FULL HIGH. THE OUTFLOW VALVE ALSO OSCILLATED TO MAINTAIN CABIN PRESS (CABIN PRESS WAS FLUCTUATING FROM +100 TO -100). AFTER DESCENDING TO 10000 FT THE NR 2 PACK FLOW VALVE INDICATED LOW WITH SELECTOR HI (PACKS-SINGLE SPEEDBRAKES OK - N503-N665). REMOVED AND REPLACED NR 2 PACK PRESSURE SENSOR (9HB) IRW A320 AMM 21-51-11-04. OPERATIONAL CHECK OF THE PRESSURE SENSOR PASSED IRW A320 AMM 21-51-00-710-002.