Boiler shows E26, sensor mismatch or implausible temperature readings between flow and return.

Safety first: Isolate power before working on sensors.

• Compare sensor readings: In the diagnostic menu (if available), compare flow and return temperatures when the boiler is off and cold; they should be similar.
• Check sensor placement: Ensure the flow sensor is on the flow pipe and the return sensor on the return. Correct any cross-connection.
• Bleed system: Remove air from the boiler and nearby pipework to ensure accurate temperature measurement.
• Replace faulty sensor: If one sensor consistently reads incorrectly compared to actual pipe temperature, replace it.

When to call a professional: If wiring or control logic issues are suspected, a technician should investigate.