Boiler display shows E0 and unit will not fire; control is unresponsive or intermittently resets

Safety first: Turn off power at the boiler service switch and at the breaker. Shut off gas at the manual gas shutoff valve before removing any covers.

• Power cycle the boiler: Turn off power for 5 minutes, then restore power and see if the E0 code clears. If it returns immediately, continue below.
• Inspect low‑voltage wiring: Remove the front panel. Locate the low‑voltage harness from the user interface to the main control board. Check for loose plugs, bent pins, or corrosion. Reseat all connectors firmly.
• Check for moisture or soot: Look for signs of water intrusion or soot on the control board. Any moisture or burn marks indicate the board should be replaced, not cleaned.
• Verify line voltage: With power ON (only if you are trained and have a meter), confirm 120 VAC at the control’s L and N terminals. Incorrect voltage or frequent brownouts can damage the board.
• If E0 persists: The main control board is likely defective. Replacement requires transferring all wiring one terminal at a time and verifying correct dip‑switch or jumper settings per the Ruud boiler service manual. If you are not experienced with boiler controls, contact a qualified technician.