A.O. Smith 300 Series Instruction Manual page 59

If tank temperature drops below Operating Set Point minus
Differential setting a heating cycle is activated
Control System performs diagnostic checks
Normal State of all pressure switches and ECO are checked
Pressure switches and ECO are verified closed
Combustion Blower is energized
Pre-Purge cycle
Spark Ignition Transformer is energized
24 V Gas Valve is energized
gas flows to Main Burner
Is flame sensed at the Main Burner ?
(control system monitors the flame sensor)
YES
Water is heated to
set point
24 V Gas Valve
de-energized
60 second
Post-Purge cycle
Water heater goes
into standby mode
NO
24 V Gas Valve
de-energized
60 second
Inter-Purge cycle
Retries up to
two more times
Control System
Locks Out
"Ignition Failure"
Fault Msg is
displayed
Figure 52. Sequence of Operation
59
Control System Locks Out
NO
Displays Fault Msg