F4 Ideal
Ideal Boiler F4 Fault: Flow Thermistor Explained
F4 on an Ideal — Logic models prominently — is the flow thermistor fault on most models: the sensor reporting the temperature of water leaving the boiler has returned a reading the control board won't accept — open circuit, short, or values outside plausibility — so the boiler has stopped trusting its own eyes and locked out.
What the F4 code means
The flow thermistor is a small resistance sensor clipped to the flow pipe; the board reads its resistance continuously to control the burn and protect the exchanger. F4 records a reading outside acceptable bounds — classically an open or short circuit from sensor failure, degraded wiring or a loose connector, and occasionally an honest report of a genuinely abnormal condition the board interprets as sensor implausibility. The boiler can't safely modulate a burner it can't measure, so it stops.
Safe steps you can try first
Sensor and pressure-sensor codes are the boiler distrusting one of its own instruments — and occasionally the instrument is right and the system genuinely has the problem it's reporting. Your safe checks: read the pressure gauge (if the boiler has an analogue one) and top up via the filling loop if it's genuinely low; check for obvious recent triggers like a radiator bleed or a system drain-down; then reset once and watch whether the code returns immediately or after running.
A code that returns instantly usually means the sensor or its wiring; one that returns after minutes of running can mean the sensor is honestly reporting a real condition — circulation, pressure fluctuation, temperature swings. Either way the diagnosis is meter-and-manual work behind the case: quick for an engineer who knows the model, and not a homeowner project.
What causes it
The suspect list is short and cheap: the thermistor itself ageing out of tolerance (the leading cause — these are wear components in the honest sense), connectors and wiring degrading with heat cycles, moisture reaching the connection, and the tail case where a real system condition (circulation faults producing genuinely wild temperature swings) presents as sensor implausibility. The diagnostic discipline matters precisely because of that tail: a competent engineer verifies the reading independently before condemning the part, so a real circulation problem doesn't get a new sensor and a return visit.
When to call a Gas Safe engineer
F4 is a book-it code: multimeter verification against the resistance table, a look at wiring and connectors, and usually a modestly priced thermistor swap — one of the quickest repairs on the Logic's engineer-friendly layout. The homeowner contribution is observational: note whether the boiler runs at all before locking out and whether heating behaved oddly beforehand (lukewarm radiators, short cycling), and report it — that's the evidence that separates sensor-lying from sensor-honest cases.
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Frequently asked questions
What does F4 mean on an Ideal boiler?
Flow thermistor fault: the sensor measuring water temperature leaving the boiler returned an implausible reading — usually the sensor or its wiring failing, occasionally an honest report of a real abnormal condition.
Is a thermistor expensive to replace?
No — it's among the cheapest boiler parts, and the Logic's layout makes the labour short. The value in the visit is the verification that the sensor, not the system, was the problem.
Can I just reset F4 and carry on?
One reset tells you whether it was transient; a returning F4 means the board still can't trust the reading, and running blind isn't on the menu. Book it — it's a quick, cheap visit.
Could F4 mean something worse than a sensor?
Occasionally — genuinely wild temperature swings from circulation faults can present as implausible readings. That's why engineers verify before replacing; mention any odd heating behaviour beforehand and the visit covers both possibilities.
Why does the code clear and come back?
Intermittent sensor faults — ageing components, wiring connections, moisture — behave exactly that way, as do genuine intermittent system conditions. Note the pattern (immediate return vs. after running) and report it; it halves the diagnostic search.
Are sensor faults expensive?
Usually not — sensors and thermistors are among the cheaper boiler components, and diagnosis with a multimeter is fast for an engineer who knows the model. The cost risk is misdiagnosis, which is why guessing at parts is a false economy.
The honest next step
Where you go from here depends on what you've worked out. A boiler that's running but grumbling can wait for a standard appointment, which is cheaper and calmer for everyone. No heat in cold weather — especially with young children, elderly or medically vulnerable residents in the house — should be booked as urgent, and you should say so when you call, because that's how triage works. Either way, describing the symptoms in the terms this guide has given you gets the right engineer with the right parts first time.