Boiler display shows E12 and unit may short-cycle or not modulate correctly

Safety first: Allow boiler to cool before handling temperature sensors.

• Locate supply sensor: Identify the supply temperature sensor on the hot outlet piping or in a sensor well.
• Inspect wiring: Check for frayed wires, loose terminals, or corrosion at the control board connection.
• Verify contact: Ensure the sensor is firmly seated in the well or tightly strapped to the pipe with insulation over it.
• Test sensor: Use a multimeter to measure resistance and compare to the manufacturer’s chart. Replace the sensor if readings are incorrect.
• If E12 persists with new sensor: The integrated control board may need replacement.