Boiler displays E04; temperature reading on the display is unrealistic (very high or very low)

Safety first: Isolate the boiler from the mains supply before opening the casing.

Steps to check:

• Locate the return NTC: On Kospel boilers, the return NTC is usually mounted on the return pipe entering the heat exchanger. Identify it by its small probe and two-wire connector.
• Inspect wiring: Follow the cable from the sensor to the control board. Look for pinched, cut, or overheated insulation. Repair or replace damaged wiring.
• Check connector seating: Unplug and replug the connector at both the sensor and the control board to ensure a solid electrical connection.
• Resistance test: With the boiler powered off, measure the sensor resistance and compare with the expected value for the current temperature (refer to Kospel documentation). Replace the sensor if readings are out of range.
• Power cycle: After any repair, restore power and reset the boiler. Confirm that the displayed return temperature is plausible and that E04 is cleared.

When to call a technician: If the error persists after sensor replacement, the fault may be on the main control board and requires professional diagnosis.