As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT 500 FEET ON FINAL, THE AUTO FAIL LIGHT ILLUMINATED ON THE PRESSURIZATION PANEL ALONG WITH GREEN ALTN LIGHT. AT GATE WE FOLLOWED THE QRH AND WHEN I MOVED THE SWITCH TO ALTN THE AUTO FAIL LIGHT WENT OUT. GROUNDED - YES. REMOVED AND INSTALLED NEW OUTFLOW VALVE, REF B737-800 AMM 21-31-03-000-801 & 21-31-03-400-80. RAN OPS CHECK, PRESSURE SYSTEM GROUND TEST PER B737-800 AMM 21-31-00-700-802, OPS NORMAL.