What the F5 code means
Your boiler measures water temperature in two places. The flow thermistor reads the hot water leaving the boiler, and the return thermistor reads the cooler water coming back from the radiators on the return pipe. Comparing the two tells the boiler how much heat the system is using, helps it run in condensing mode as efficiently as possible, and lets it spot circulation problems.
When the return thermistor reading disappears or falls outside its expected range, the boiler loses that comparison and shuts down with F5. In Ideal's installation manuals for the Logic Code Combi, Logic+ Combi and Logic Combi2, the fault is listed as F5 or L5, return thermistor fault. As with most Ideal codes, the F version can clear once the reading returns to normal, while L5 is a lockout that needs a reset.
Ideal code lists differ slightly between ranges and generations, so if you have a Logic Max, Vogue or an older model, check the table in your own manual before assuming F5 means a return sensor fault.
Common causes of F5
The return thermistor has failed
This is the most common cause. The sensor is clipped or fitted to the return pipe inside the boiler, and like any electronic part it can fail with age. Ideal's servicing instructions have engineers disconnect it and measure its resistance to confirm.
Wiring or plug problems
A loose plug, a corroded terminal or a wire that has chafed against metal can all give a false reading. Faults of this type are often intermittent, appearing and disappearing for no obvious reason.
Moisture near the sensor
A weeping joint or a small leak inside the boiler can reach the sensor or its connector. If you have noticed damp under the boiler or pressure dropping, tell the engineer. Our guide on a boiler leaking water explains what to look for.
Control board fault
Occasionally the thermistor is fine and the problem lies on the control board. An engineer will only reach this conclusion after testing the sensor and wiring.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Failed return thermistor | F5 or L5 returns every time after a reset | Gas Safe engineer |
| Loose plug or damaged wire | Intermittent fault, often after the boiler warms up | Gas Safe engineer |
| Moisture from a small leak | Damp under the boiler or pressure dropping | Gas Safe engineer |
| Control board fault | Sensor and wiring test correctly | Gas Safe engineer |
Safe checks you can do yourself
You cannot fix a sensor fault from outside the boiler, but these steps may clear a one-off glitch and will help your engineer.
- Check the fused spur is switched on and that the rest of the house has power.
- Restart the boiler once. Press reset as the manual describes, or turn the boiler off at the fused spur, wait a minute and turn it back on.
- Run the heating and hot water separately and note whether the fault appears with one, the other or both.
- Check the pressure gauge reads 1 to 1.5 bar with the system cold. If it is low, top it up using the filling loop, and read our guide on boiler pressure that is too low.
- Look under the boiler for drips, without opening the casing.
- If the code returns after two or three attempts, stop and book an engineer.
Do not remove the boiler casing to find or test the thermistor. Testing sensors means working near live connections and hot pipework, which is a job for a Gas Safe registered engineer.
When to call a Gas Safe engineer
If F5 or L5 keeps coming back, you need an engineer. The fix is often quick: they test the sensor's resistance against the values in Ideal's manual, check the wiring and replace whichever part has failed. Tell them the exact code and whether the fault is constant or comes and goes.
While the engineer is there, it is worth asking them to check the flow thermistor and the system pressure as well. The two sensors age at a similar rate, and the boiler relies on both to judge how well water is moving round the system. If circulation is poor, the boiler may also show an L1 lockout, so mention any other codes you have seen recently, even if they cleared on their own.
If your boiler is still under warranty, contact Ideal or your installer before booking anyone else, so you do not affect your cover.
Typical repair costs in the UK
These are typical ranges including parts and labour. Your price depends on your model, your area and whether the engineer carries the part.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Return thermistor replacement, fitted | Typically £90 to £180 |
| Repairing wiring or a connector | Typically £80 to £150 |
| Fixing a small internal leak | Typically £100 to £250 |
| Replacement PCB | Typically £300 to £550 |
How to stop F5 coming back
Once a failed sensor has been replaced, F5 rarely returns quickly. A yearly service is the best way to catch loose connections, early leaks and heat damage to wiring. Our boiler service cost guide covers what to expect. Keep an eye on the pressure gauge too, because a system that keeps losing pressure often has a leak somewhere, and water inside a boiler causes a range of electrical faults. If you see a flow sensor fault instead, read our page on the Ideal F4 flow thermistor fault.