Boiler display shows “Err 8” and burner will not start, often accompanied by a very high temperature reading

Safety first: Allow the boiler to cool and shut off power. Do not open any piping while hot or pressurized.

• Check actual temperature: Carefully touch the outlet pipe with the back of your hand (without contact with bare metal). If it is only warm but the display shows extreme temperatures, the sensor is likely faulty.
• Inspect sensor wiring: Trace the outlet sensor wires and ensure they are not touching the flue, burner, or heat exchanger where insulation could melt and cause a short.
• Verify circulator operation: If the outlet pipe is extremely hot, the circulator may not be moving water. Check that the pump runs on a call for heat and that valves are open.
• Replace sensor if damaged: Install a new outlet temperature sensor if the old one shows physical damage or Err 8 persists.
• Reset and monitor: Restore power, reset the control, and monitor temperatures. If overheating continues, shut down and call a technician to check flow and control logic.