Gas control valve is not identified, so error code is not known.
ECO means energy cut off.
Energy cut off happens when gas control valve thermostat has gone bad, or when gas control senses runaway heating inside combustion chamber.
Newer heaters have resettable ECO for combustion overheat. This reset is typically located on combustion chamber door and is round and has two wires that connect back to gas control. Generally you have to clean combustion chamber and tubes and catch up on maintenance, then reset ECO, then fire up heater again to see if problem is resolved.http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoothttp://waterheatertimer.org/images/Typical-gas-valve-500.jpg
Newer heater does not have reset for bad thermostat, and in that case you replace gas control valve thermostat.
Older heater does not have reset, and combustion chamber and parts are cleaned, and gas control valve thermostat is replaced.
Your manual is best place to start troubleshoot.
Open following link to help identify gas control valve:http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html