Boiler shows E05, heating and/or hot water stop, temperature readings appear abnormal.

Safety first: Isolate the boiler from the mains before removing any covers. Sensors are low-voltage but live parts are nearby.

User-level checks:

• Reset the boiler and see if the error clears temporarily.
• Ensure that no external controls (room thermostat, programmer) are calling erratically, which might confuse diagnosis.

Technical checks (qualified personnel):

• Locate the flow NTC sensor on the primary flow pipe leaving the heat exchanger.
• Disconnect the sensor and measure its resistance at a known water temperature; compare with Chaffoteaux NTC resistance tables.
• Inspect the wiring harness and connector for corrosion, loose pins, or damage; repair or replace as needed.
• If resistance is out of range or intermittent when flexed, replace the flow NTC sensor.
• Check the control board input for correct voltage and continuity.

After replacing the sensor, clear the fault by resetting the boiler and verify stable operation.