A.O. Smith ProLine / Signature and State atmospheric gas tanks: quick sheet (diagnostic and electrical side)

The pilot heats a thermopile that powers the electronic gas control, so the heater needs no electrical connection. The status LED gives a blink code repeated every three seconds; the control's built-in timing makes you wait between lighting attempts (the State handbook calls for about 10 minutes) so the thermopile can cool and the control can reset its checks.

A sensor in the control's well reads tank temperature. When the water cools below the knob setting the control opens the main valve and the pilot lights the burner. If the water overheats, the control's energy cut-off locks out the burner and shows 4 flashes; A.O. Smith's own help pages tell owners to shut the gas off and get service rather than reset it.

The FVIR system uses a flame arrestor at the bottom of the sealed chamber and a resettable thermal (TCO) switch, usually mounted on the manifold/burner door and wired in series with the thermopile circuit. The switch opens if the chamber gets too hot from flammable vapors, a blocked vent, poor draft or not enough combustion air. A.O. Smith's support pages describe a small reset button in the center of this switch; the service handbook says to check for vapors, combustion air and draft before relighting, and to inspect the flame arrestor for discoloration (a sign of a vapor event) before lighting.

Water side: cold enters through a dip tube to the bottom; hot leaves from the top. ProLine units are the plumber-grade line and Signature is the retail line; a contractor comparison (single source) reports Signature units may use plastic drain valves where ProLine uses brass. Model numbers and first-hour ratings differ between GCR and GCG even at the same tank size and 40,000 BTU input, so always size and order parts from the rating plate.