Ideal Boiler F5 - Return Thermistor Fault

What does this error code mean?

Symptoms users may notice

1. The boiler displays F5 or alternating F and 5.
2. Heating or hot water may be unavailable while the fault is active.

What users often notice

1. Record whether the code returns after the permitted reset to help the engineer.

Why is this happening?

1. A return-temperature sensing fault
2. A fault in the return-sensor circuit requiring diagnosis
3. A model-specific condition requiring the correct Ideal manual

Component context and limits

Safe next steps

1. Record the full boiler model and displayed characters.
2. Use one standard homeowner reset only as described in that model’s manual.
3. If F5 remains or returns, stop resetting and contact Ideal or a suitably qualified Gas Safe registered engineer.

What not to do

1. Do not repeatedly reset a returning fault.
2. Do not remove the boiler casing or attempt sensor replacement.

Common misdiagnoses

1. Treating F5 as a generic instruction to replace any boiler sensor.

Reset steps

1. Follow the normal user reset described in the exact model manual.
2. If the warning persists or returns, arrange service instead of repeating the reset.

Manual reference

Frequently asked questions