AFTER RELEASING A B737-500 WITH A CONTINUING MEL FOR A CHRONIC CABIN PRESSURIZATION 'AUTO FAIL' LIGHT; MECHANIC WAS INFORMED HE SHOULD HAVE GROUNDED THE AIRCRAFT AND PERFORMED A PRESSURIZATION HEALTH CHECK PRIOR TO DEPARTURE.

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WHEN REVIEWING HISTORY AND MEL FOR ACFT PRESSURIZATION; SAW NUMEROUS COMPONENTS R&R'ED. INITIALLY THOUGHT POSSIBLE WIRING PROB. TESTED PER MAINT MANUAL 21-31-00; P501 AND PERFORMED BITE CHK OF BOTH CPC'S; SYSTEM OK. FOLLOWED TROUBLESHOOTING PROC PER 21-31-00. ALL CHKED GOOD. THIS SECTION SUGGESTS DOING PRESSURIZATION HEALTH CHK AS PER 21-30-00. UNABLE TO ACCOMPLISH DUE TO MANPOWER. TABLE 'B' STATES THAT AT THIS POINT CABIN PRESSURIZATION CTL SYSTEM OK. FLT FAULT (BOTH CPC'S) SHOWED LOW FLOW/HIGH LEAK; CODE 30. THIS SAYS THAT THE OUTFLOW VALVE WAS BELOW 3 DEGS FOR LONGER THAN 5 SECONDS. DUE TO HISTORY; RECOMMENDED MEL REMAIN. IF ACFT HAD PASSED PRESSURIZATION HEALTH CHK; THEN WIRING OR OTHER INPUTS SUCH AS A RELAY OR DIGITAL AIR DATA COMPUTER AS POSSIBLE SUSPECTS. THIS ACFT HAD AN EXTENSIVE HISTORY OF FAILING CABIN PRESSURIZATION CTL IN AUTO MODE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE MANUAL AC; MANUAL DC AND STANDBY POSITION PRESSURIZATION CONTROLS WORKED FINE ON THE B737-500. BUT IN THE AUTO MODE POSITION; THE 'AUTO FAIL' LIGHT ILLUMINATING WAS A CHRONIC PROBLEM. THE AIRCRAFT WAS GROUNDED AT A DOWNLINE STATION; WHERE THE PRESSURIZATION HEALTH CHECK WAS ACCOMPLISHED. REPORTER STATED HE WAS TOLD A PLASTIC TIE WRAP WAS FOUND RESTRICTING THE MAIN (AFT) OUTFLOW VALVE. REPORTER STATED THAT DIDN'T MAKE ANY SENSE; BECAUSE THE PRESSURIZATION SYSTEM WORKED FINE IN THE MANUAL AND STANDBY MODES AND WAS ORIGINALLY DEFERRED BECAUSE THEY WERE WORKING FINE; EXCEPT FOR THE AUTO POSITION. REPORTER STATED THE MAIN (AFT) OUTFLOW VALVE WAS REPLACED AND DIRT WAS FOUND IN THE AFT CARGO DRAIN VALVE.