Tankless unit displays E2 and shuts down shortly after starting; water may get briefly warm then go cold.

Safety first: Turn OFF electrical power and close the gas valve. Allow the unit to cool completely before servicing.

• Step 1 – Check water flow: Make sure all inlet shutoff valves to the heater are fully open. Clean the cold water inlet filter screen on the bottom of the unit (shut off water first, remove the filter, rinse debris, reinstall).
• Step 2 – Inspect for scale: If the heater has not been descaled in the last 6–12 months (or sooner in hard water areas), mineral buildup is likely. Connect a pump, hoses, and a bucket with manufacturer-approved descaling solution to the service valves and circulate through the heat exchanger for 45–60 minutes. Flush thoroughly with clean water afterward.
• Step 3 – Check thermistors: Locate the outlet thermistor and overheat sensor on the heat exchanger. Inspect wiring for damage or loose connectors. Reseat connectors firmly.
• Step 4 – Reset and test: Restore power and gas, then run hot water. If E2 persists, the outlet thermistor or overheat sensor may be defective.
• Step 5 – Replace sensor(s): With power and gas OFF, remove the mounting screw or clip holding the thermistor/overheat sensor, disconnect the wiring, and install new OEM Ruud sensors. Reassemble and test.

If the error continues, the control board or gas valve modulation may require professional diagnosis.