As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE OPERATING IN ALTN MODE FOR PRESSURIZATION UNDER ASA MEL 21-14C THE AUTO FAIL LIGHT ILLUMINATED DURING CLIMBOUT AT FL240. OPERATED REMAINDER OF FLIGHT UNDER MAN MODE. PRESSURIZATION WAS FLUCTUATING AND HARD TO CONTROL. GROUNDED - YES. REMOVED AFT OUTFLOW VALVE ASSY. REF B737-900 AMM 21-31-03-000-801. INSTALLED NEW OUTFLOW VALVE PER AMM 21-31-03.