F4
Saunier Duval Primary Flow Temperature Sensor Fault (NTC)

Boiler displays F4 and stops heating, often after running for a short time

Advertisements

Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Advertisements

Possible Causes

Open-circuit or shorted flow NTC sensor, Poor contact at NTC connector, Limescale build-up causing local overheating at sensor pocket

How to Fix / Troubleshooting

Safety first: Isolate power and allow the boiler to cool fully before touching any pipework or sensors.

Steps to check:

  • Remove the front panel and locate the primary flow NTC sensor on the flow pipe leaving the main heat exchanger (usually clipped or screwed into a pocket).
  • Inspect the sensor wiring and plug for damage or loose connection. Reseat the connector.
  • With power off, disconnect the sensor and measure its resistance with a multimeter. Compare to the Saunier Duval temperature/resistance chart.
  • If the reading is open, shorted, or far from expected, replace the NTC sensor.
  • If the sensor is in a pocket, check for heavy scale or sludge; flush the system if necessary (engineer task).
  • Refit, restore power, and reset the boiler to see if F4 clears.

Warning: Draining the boiler or system may be required to replace some sensors. If unsure, call a heating professional.

Advertisements

Repair Difficulty

Medium 3/5

Required Part

Primary Flow NTC Sensor
Check Price on Amazon

We may earn a commission from links on this page.

Advertisements