Light flashing - lockout condition - ECO failure

The ECO is designed to shut off the water heater in case of the water heater overheating. This is to prevent the water heater from exploding. I assume that you have a newer style residental water heater or a commerical boiler. The ECO is a fuseable link that is built into the control and located at the thermostat probe. If the water heater overheats then this link will melt cutting the circuit on the millivolt system(residential) or 24volt system(commercial boiler). If you look under the water heater control(where the temp adjustment is) there are usuall two wires that go from the control to the water heater. These are the ECO wires. Verify the wires have not been cut or damaged. If these wires are cut or damaged then the ECO circuit will be incomplete and you will get the ECO lockout failure. Also make sure that the flame lock-out switch is not tripped. It will be located at the burner compartment cover and have two wires going to it. Most have a little button in the center. These are to sense high heat and one might have tripped closing the circuit. You can push this little button to re-set the switch. If you feel a little "click" then the switch was probably tripped. If it does not have any button then it might be "self resetting". To test that switch use a continuity tester and the circuit should be closed. If these switches are working then the fusable link for the ECO is probably bad from the water heater temp getting too high or was defective and failed. This will require you to remove the water heater control valve and replace the fusable link. If you do not have experience with this I suggest you call a licensed plumber to make the repairs. If this repair is improperly done it could void the warranty on the water heater and/or cause property/personal damage or injury. There is a possiblilty that your water heater thermostat had failed causing the ECO to trip. If that is the case then the ECO safety did it's job.