First, thanks for all of the suggestion.
Second, I after re-reading my original post, I realize I did NOT describe the symptoms ! The water heater lights as soon as you turn it on, indicating that the ECO and thermostat switches, valve and ignition are all working. The problem occurs AFTER the water has been heated and it is cooling down.
The igniter comes on, indication that the thermostat switch has closed, but the burner does NOT ignite. I ASSUME this is because the ECO switch is open (ie. the gas vales are closed).
EDIT : I forgot to say, after trying to ignite and failing, the control board shuts everything down and light the "failed" light on the control panel. Of course, by the time you notice this the water is cold. If you cycle the power switch then, the burner DOES come on.
I will have to get a thermometer, some hot water and test each one with my DMM.
