You have AO Smith tank-type power vent water heater with FVIR flame arrestor.
1) I believe this system has White Rodgers intellivent gas valve, with led indicator lights.
To test if valve is operational:
-NORMAL operating mode displays no lights on gas valve.
-Unplug unit from electrical outlet and plug back in. All lights on gas control should display for 1-5 seconds.
-Press BOTH Hotter and Colder temperature buttons for one second to 'wake up' the control, and release buttons and LED will display temperature.
-Press Hotter or Colder and see if any indicator light shows on gas valve.
This will give baseline if gas valve is functioning.
-Intellivent gas valve requires outlet that is wired 100% correctly. Test outlet with circuit analyzer.
-Water heater must be properly grounded for intellivent to operate.
2) Open following link to identify gas valve and download possible service manual with error codes and testing procedure. Service manual is a large file and will probably not load on dial-up.http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html#intelli
3) Open following link for AO Smith manuals and service handbooks.
Shows same intellivent service manual, and shows manuals for power vent, but does not specifically list the GPS75 serieshttp://www.hotwater.com/resources/product-literature/service-handbooks/
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