Boiler control shows “Err 6” and will not start, often after a power outage or wiring work

Safety first: Turn off power before touching any wiring or control boards.

• Locate temperature sensors: On Burnham modulating condensing boilers, the supply and return sensors are usually clipped or inserted into wells on the piping near the heat exchanger. Identify the sensor referenced by the manual for Err 6.
• Inspect wiring: Follow the sensor cable back to the control board. Look for cuts, melted insulation, or loose connectors. Reseat the plug firmly on the control.
• Check for shorts or damage: Ensure the sensor wires are not pinched under the boiler cover or touching hot flue components.
• Compare readings (if display shows temp): If the control displays an obviously wrong temperature (e.g., -40°F or 250°F when the boiler is cold), the sensor is likely failed.
• Replace sensor: Shut off power, remove the old sensor from its clip or well, and install a factory Burnham replacement sensor of the same type. Use thermal paste if specified.
• Restore power and test: Turn power back on and verify the displayed temperature is reasonable and Err 6 clears. If the error persists with a new sensor, the control board may be defective and should be evaluated by a technician.