F5, or L5 on some Ideal displays, is a return thermistor fault. The sensor on the central heating return pipe is not giving a usable reading.

Same family as F4, and treated the same way. It is a sensor, not a system problem.

What F5 or L5 actually means

The boiler compares flow and return temperatures to work out how hard the system is working and whether water is actually circulating.

Without a trustworthy return reading it cannot do that, so it stops.

What you can safely check yourself

Nothing useful, the sensor is on the return pipe inside the appliance.

If you have had an overheat lockout recently as well, mention it, because a faulty return thermistor can cause one.

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

Can a bad return thermistor cause an overheat lockout?

Yes. If the boiler cannot see the return temperature correctly it can misjudge the system and trip its overheat protection.

Is it an expensive repair?

A thermistor is a modest part. You will get a price before work starts.

Is F5 the same as L5?

Yes, the same fault presented differently depending on the display.

Related fault codes

Other Ideal codes: L1 · L2 · LC · L6 · F1 · F2 · F3 · F4 · F6 · F7 · F8 · F9 · all boiler advice. See also boiler repair. See also Ideal L1 overheat lockout.

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.