Boiler displays E05 and will not fire; may occur immediately on power-up or during operation.

Safety: Turn off power before accessing internal components.

• Locate the supply sensor: On IBC boilers, the supply temperature sensor (NTC) is typically clipped or threaded into the supply outlet or heat exchanger manifold. Refer to the unit's diagram.
• Inspect wiring: Check the two-wire harness from the sensor to the main control board. Ensure the connector is fully seated and wires are not pinched or damaged.
• Check sensor mounting: If the sensor is clipped to a pipe, make sure it is firmly attached with thermal paste or proper contact. Loose sensors can give erratic readings.
• Reset and test: Restore power and clear the error. If E05 returns immediately, the sensor is likely defective.

Replacing the supply temperature sensor is usually straightforward but must be done with the correct IBC part. If a new sensor does not resolve the issue, the control board may require replacement by a professional.