Error F4

Safety first: If you smell gas, leave the property immediately and call the gas emergency number. Do not attempt internal checks if you are not competent with electrical appliances. Always isolate the mains electrical supply to the boiler before removing the case or touching internal components. Avoid working on gas valves, burners or carrying out repairs that require a gas qualification.

Initial homeowner checks you can do without opening the boiler:

1) Note the display and any other codes shown. Try a single restart using the boiler’s RESTART/reset button following the manual instructions. Do not repeatedly reset more than once or twice. 2) Check the system pressure gauge — aim for about 1.0–1.5 bar when cold. If pressure is very low, repressurise per the manual and then try a reset. 3) Confirm external controls (programmer/timers, room thermostat, cylinder stat) are calling for heat. 4) Listen for the central heating pump running when there is a demand: if the pump is completely silent it may be seized or powered off, causing no flow. 5) In cold weather check condensate pipes and visible flow pipes for freezing. 6) Look for obvious water leaks or corrosion around pipe unions and the boiler base.

If you are comfortable and it is safe to proceed (isolate mains first) you can do these diagnostic checks:

1) Remove the outer cover only if you are confident and the boiler is isolated. Locate the flow thermistor: it is usually a small probe clipped to the flow pipe with a 2‑wire push-on connector back to the PCB. 2) Visually inspect the connector and wiring for loose push-on connectors, corrosion, water ingress, crushed/shorted wires. Re-seat the connector securely. 3) If you have a multimeter and know how to use it safely: with the boiler isolated, measure continuity between the two thermistor wires at the connector. A completely open circuit (infinite resistance) or a dead short (near 0 ohms) indicates a faulty thermistor or shorted wiring. Thermistor resistance values vary by part and temperature, so lack of continuity or a direct short is the key sign rather than an exact number. 4) If possible also measure at the PCB connector to confirm the wiring between thermistor and control board is intact. Check for any short to earth/ground. 5) If wiring and connector look good but the fault persists after re-seating and checking, the thermistor itself is likely faulty and needs replacement.

Specific fixes and next steps:

- After re-seating connectors and confirming system pressure and pump operation, power the boiler back on and attempt one reset. If F4 clears and the boiler runs normally, monitor for recurrence — intermittent faults usually indicate a failing sensor, intermittent wiring fault or water ingress. - If isolating/re-seating does not clear F4, do not attempt component-level gas-side repairs. Replacement of the flow thermistor, further electrical testing or control PCB replacement should be carried out by a qualified Gas Safe/registered engineer. The engineer will also check circulation (pump operation and direction), any external pumps/valves, and whether a PCB fault is giving a false sensor error.

When to call a professional: Call a Gas Safe registered engineer (or your local qualified installer) if the F4 code remains after basic checks and a reset, if the thermistor shows open/short on testing, if there is evidence of leaks or corrosion, if the pump is not running, or if you are not confident performing the safe isolation and checks. Do not attempt to replace the thermistor or work inside the boiler if you are not qualified; gas and electrical safety regulations require a competent engineer for most internal repairs.