- Boilers
- Gas boiler
F4
Ideal F4 on a gas boiler: flow thermistor fault
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On Ideal boilers, F4 is a flow thermistor fault: the sensor that reads heating flow temperature is giving a bad signal. Ideal says to reset once; if F4 remains, contact Ideal or a Gas Safe registered engineer quoting F4.
Safety first
Stop and check this first
- If you suspect a gas leak: put out any naked flame, do not operate electrical switches, open windows and doors, leave the home and call the National Gas Emergency Service on 0800 111 999 from outside, as Ideal's guide gives the number.
- Ideal sends F4 to its engineers or a Gas Safe registered engineer after one reset: do not open the casing to look for the sensor.
Safe checks you can perform
- Display and pattern
- One reset
- Pressure gauge
- Thermistor measurement
Leave these tasks to a qualified professional
- Do not attempt this professional task: Wiring and connector check
- Do not attempt this professional task: Safety checks after closing the case
- Do not attempt this professional task: Thermistor resistance test
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Wiring and connector check
What they doThe engineer inspects and tests the thermistor wiring and plug back to the PCB.
They must show youthe connector condition.
In writingwhat was cleaned, remade or replaced.
Safety checks after closing the case
What they doOnce the casing is refitted, the engineer runs the boiler and checks combustion and the flue.
They must show youthe boiler heating without F4 and the analyser reading.
In writingthe checks done.
Thermistor resistance test
What they doWith the power off, the engineer measures the flow thermistor and compares it with the manufacturer's data at the measured temperature.
They must show youthe measured and expected values.
In writingboth values and any part replaced with its reference.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Display and pattern
- Who:
- you.
- How:
- note whether F4 (or L4 on some models) shows steadily or comes and goes.
- Result that matters:
- whether the fault is constant.
- If it comes and goes:
- a connection fault is more likely; tell the engineer.
- If it is constant:
- the thermistor is the first suspect.
- Evidence to keep:
- photos of the display with dates.
Evidence: Ideal Instinct2 user guide · Ideal Heating FAQ: what does my boiler fault code mean?
One reset
- Who:
- you.
- How:
- press the restart button once, as Ideal advises for F4.
- Result that matters:
- whether F4 clears.
- If F4 returns:
- contact Ideal or a Gas Safe engineer quoting F4.
- If it clears:
- note it; repeats confirm an intermittent fault.
- Evidence to keep:
- a note of each reset.
Evidence: Ideal Instinct2 user guide · Ideal Heating FAQ: what does my boiler fault code mean?
Pressure gauge
- Who:
- you.
- How:
- read the system pressure gauge; Ideal asks for 1 to 1.5 bar.
- Result that matters:
- whether the boiler also lacks water.
- If it is low:
- top up as your user guide describes and mention it.
- If it is correct:
- the fault stays with the thermistor circuit.
- Evidence to keep:
- a photo of the gauge.
Evidence: Ideal Instinct2 user guide · Ideal Heating FAQ: what does my boiler fault code mean?
Thermistor measurement
- Who:
- Ideal or a Gas Safe registered engineer.
- How:
- measures the thermistor and checks its wiring and plug.
- Result that matters:
- whether the sensor reads correctly for its temperature.
- If the sensor reads wrong:
- it is replaced.
- If it reads right:
- wiring or board are checked.
- Evidence to keep:
- the measured value written down.
The thermistor is inside the boiler casing with mains and gas parts; only Ideal or a Gas Safe registered engineer opens it.
Evidence: Ideal Instinct2 user guide
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Ideal Heating
- Product type
- Gas boiler
Known code variants
- AEG F4 error fridge freezer
- AEG F4 frigorífico
- AEG fridge F4
- AEG fridge freezer F4 temperature sensor
- caldera Viessmann F4
- DishDrawer F4
- error F4 DishDrawer
- error F4 frigorífico AEG
- error F4 Viessmann
- F 4
- Fisher & Paykel DishDrawer F4
- Fisher Paykel F4
- frigorífico AEG F4 sensor temperatura
- Ideal boiler F4
- Ideal boiler F4 flow thermistor
- Ideal F04
- Ideal F4
- Ideal L4
- Ideal Logic F4
- Viessmann boiler F4
- Viessmann F4
- Vitodens F4
What it means
Ideal's FAQ lists F4 as flow thermistor, and the Instinct2 user guide shows the same fault as F4 or L4. The flow thermistor is a temperature sensor on the heating flow; the boiler uses it to control and protect against overheating, so it stops when the reading is implausible. The code does not tell a failed thermistor from its wiring, a poor connection or the board reading it; those checks need an engineer with a meter.
Warnings and stop conditions
- Criticaleditorial warning
If you suspect a gas leak: put out any naked flame, do not operate electrical switches, open windows and doors, leave the home and call the National Gas Emergency Service on 0800 111 999 from outside, as Ideal's guide gives the number.
Evidence: Ideal Instinct2 user guide · HSE gas safety for homeowners
- Mediumsource verified
Ideal sends F4 to its engineers or a Gas Safe registered engineer after one reset: do not open the casing to look for the sensor.
Evidence: Ideal Instinct2 user guide · Ideal Heating FAQ: what does my boiler fault code mean?
Symptoms and causes
Probable causes · in order
- Thermistor failedPossible
Fits if F4 stays after a reset and the sensor measures out of range when tested.
Evidence: Ideal Instinct2 user guide · Ideal Heating FAQ: what does my boiler fault code mean?
- Wiring or connectionPossible
Fits if F4 comes and goes; a loose or corroded plug gives the same code as a dead sensor.
Evidence: Ideal Instinct2 user guide
- Control boardLess common
Fits if sensor and wiring test correct; the board's reading is then checked.
Evidence: Ideal Instinct2 user guide
Virtual technician
Does F4 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Ideal Heating · 2023 · manual
- 2
Health and Safety Executive · official_doc
- 3
Ideal Heating FAQ: what does my boiler fault code mean?
Ideal Heating · official_doc
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