Boiler attempts to start but shuts down with E02; fan may run briefly, no sustained flame

Safety: Switch off power at the breaker and allow the boiler to cool before opening the cabinet.

• Inspect flame sensor: Locate the flame rod on the burner assembly. Remove the mounting screw and gently pull it out. Clean the metal rod with fine steel wool or emery cloth until shiny. Do not sand the ceramic insulator. Reinstall and ensure it is fully inserted into the flame path.
• Check ground connections: Verify the green ground wire from the boiler chassis to the electrical junction box is tight and corrosion‑free. Tighten any loose screws on the burner ground strap.
• Verify line polarity: At the boiler’s power connection, confirm hot and neutral are not reversed (use a plug‑in outlet tester or multimeter). Correct wiring if needed.
• Reset and test: Restore power, reset the boiler once, and observe ignition. If E02 persists, the control board flame circuit may be defective and should be tested and replaced by a qualified technician.