Boiler shows F1 and does not fire, no heating or hot water

Safety: Isolate electrical power and close the gas supply before opening the boiler casing. Allow the appliance to cool.

Diagnosis steps:

• Locate the return NTC sensor on the return pipe entering the heat exchanger or hydraulic block. It will look similar to the flow NTC but mounted on the cooler return side.
• Check the sensor body for cracks, water ingress, or signs of overheating. Ensure the clip or bracket holds it firmly against the pipe or in its pocket.
• Unplug the sensor and measure its resistance with a multimeter at room temperature. Compare with the Protherm NTC chart. Replace the sensor if the reading is open, shorted, or significantly off‑spec.
• Inspect the wiring harness from the sensor to the PCB for cuts, crushed insulation, or signs of rodent damage. Repair or replace as needed.
• Clean the connector pins with electrical contact cleaner if they appear oxidized.
• If sensor and wiring are OK, the PCB input channel may be defective. A qualified technician should test the PCB and replace it if necessary.

After work: Refit all covers, restore power and gas, reset the boiler, and run a heating cycle to verify that the F1 code no longer appears and that return temperature is being read.