The display shows A8, and the furnace does not start the heating cycle.

Safety first: Disconnect electrical power and allow the furnace to cool before working on pipework or sensors.

• 1. Locate return NTC: The return temperature sensor is usually mounted on the return pipe entering the heat exchanger or hydraulic block. Consult the Junkers service diagram.
• 2. Inspect sensor and wiring: Look for moisture, corrosion, or physical damage. Ensure the sensor is firmly clipped or screwed into the pipe and that the connector is tight.
• 3. Test the sensor: Remove the connector and measure resistance with a multimeter at ambient temperature. Compare it with the Junkers NTC resistance chart. Replace the sensor if readings are out of tolerance, open, or shorted.
• 4. Check cable and plug: Inspect the cable for cuts or crushed sections. Verify continuity from the sensor plug to the control board. Repair or replace as needed.
• 5. Reset and test: After replacing or reseating the sensor, restore power, reset the furnace, and check if the A8 code clears and normal operation resumes.
• 6. Control board diagnosis: If the sensor and wiring are confirmed good, the main PCB may be faulty. This should be diagnosed and replaced only by an authorized Junkers technician.