Tankless water heater powers on but does not fire; display shows E0 and unit will not enter normal operation.

Safety first: Turn OFF power at the breaker and close the gas shutoff valve before removing any covers. Allow the unit to cool completely.

• Step 1 – Power reset: Turn the unit OFF, switch the breaker OFF for 5 minutes, then restore power and see if the E0 clears. If it does not, continue.
• Step 2 – Check DIP switch settings: Remove the front cover. Locate the main control board (PCB) and identify the DIP switches. Compare their positions with the Ruud installation manual for your exact model (gas type, altitude, recirculation, etc.). Correct any mis-set switches and power cycle the unit.
• Step 3 – Inspect harness connections: Verify all multi-pin connectors on the control board are fully seated and free of corrosion or moisture. Reseat each connector one at a time.
• Step 4 – Replacement board verification: If a control board was recently replaced, confirm the part number exactly matches the Ruud model and that any required programming or jumper settings were performed.
• Step 5 – Replace control board: If E0 persists after configuration is verified, the PCB may be defective. Label and disconnect all harnesses, remove mounting screws, and install a new OEM Ruud main control board. Reassemble, restore power, and test.

Call a professional if you are not comfortable working around live electrical components or if configuration information is not available.