Boiler displays E05 and will not fire; flow temperature reading is implausible or fixed.

Safety: Isolate electrical power before accessing internal components.

• Reset and check display: Reset the boiler and observe the indicated flow temperature. If it shows a constant extreme value (e.g. -40°C or 130°C), the NTC is likely faulty.
• Inspect sensor connection: Remove the front cover and locate the flow NTC on the primary flow pipe or heat exchanger. Check the plug connection is fully seated and free of corrosion.
• Check wiring: Follow the harness from the NTC to the PCB. Look for damaged insulation, pinched cables, or loose connectors. Repair or replace the harness if needed.
• Test NTC resistance (technician): Disconnect the NTC and measure its resistance with a multimeter at room temperature. Compare with CTC specification (typically around 10 kΩ at 25°C). Replace the flow NTC sensor if out of range or open/shorted.
• PCB check (technician): If a known-good NTC still gives E05, the analogue input on the main PCB may be defective. Replace the main control PCB following CTC procedures.