F4, or L4 on some Ideal displays, is a flow thermistor fault. The sensor that tells the boiler how hot the water leaving the heat exchanger is has failed or lost its connection.

This is a component fault rather than a system fault, and it is one of the more straightforward ones on the list.

What F4 or L4 actually means

The flow thermistor sits in the top of the heat exchanger. Every decision the boiler makes about firing and modulating depends on the reading it gives.

If that reading is missing, out of range or implausible, the boiler stops rather than run blind.

What you can safely check yourself

Nothing useful. The thermistor is inside the appliance.

Worth noting whether the fault shows from cold or only once the boiler has warmed up, because that is genuinely useful to whoever attends.

When to stop, and why

What causes it beyond the simple checks

The manufacturer’s chart has the engineer check the wiring is securely connected, then disconnect it and measure the thermistor resistance against published values, roughly 9,700 to 10,300 ohms at 25°C, 2,400 to 2,600 at 60°C and 1,000 to 1,100 at 85°C. If the value is wrong the thermistor is replaced. If it is right, the wiring to the PCB is checked next, and the board last.

How Vantage can help

We are Gas Safe registered under 956889 and cover Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day. Call us on 01462 506374. See also boiler repair and boiler servicing.

Frequently asked questions

Is F4 serious?

It is not a combustion or overheat fault, but the boiler will not run until it is sorted.

Will I need a new thermistor?

Not necessarily. A loose or corroded connection gives the same code and is checked first.

What is the difference between F4 and L4?

Different Ideal displays present the same fault either way. It is the same thermistor.

Related fault codes

Other Ideal codes: L1 · L2 · LC · L6 · F1 · F2 · F3 · F5 · F6 · F7 · F8 · F9 · all boiler advice. See also boiler repair.

In Ideal’s own words

Ideal use different code sets across their ranges and generations. The definition above comes from The Ideal installation and servicing guide. If your boiler is a different model and the code behaves differently, check the manual for your appliance or ring us and we will tell you what it means on your unit.

When to call an engineer

If the checks above have not cleared it, the next steps are engineer work and the boiler should not be reset repeatedly in the meantime. Call Vantage Plumbing and Heating on 01462 506374. We are Gas Safe registered under 956889, the line is answered by a person at any hour of any day, and you get a price before anyone starts.

Need help today?

Call and speak to a person, any hour, any day of the year. Get a fixed price quote or call 01462 506374.