Noritz tankless water heaters are meant to supply hot water for a residence without the use of a large storage tank. The water heater unit is installed on a wall outdoors, and offers features such as a safety lock-out set point, automatic overheat protection, direct ignition, automatic air regulation and resin coated PC boards. The unit also has an electronic control panel where error codes may appear if there is a problem with the unit. An error code "11" typically signifies an ignition error.
Find the gas valve connected to the Noritz water heater and turn the gas valve counterclockwise until it no longer turns to open it completely.
Turn the power off to the unit by pressing the "Power" button. Run hot water from a nearby faucet. Wait 5 minutes and press the "Power" button again to restore power to the water heater.
Turn the hot water back off. This should resolve the problem.