What to do
- Reset the boiler
Press the restart button.
When to call a Gas Safe engineer
If the code comes back, contact Ideal quoting F4 (or L4): it needs an engineer visit. Outside the warranty, use a Gas Safe registered engineer.
A repair visit will usually check the sensor's reading and its wiring.
- Note the model and serial number. They're on a label on the data badge, usually under the boiler or behind the front cover.
- Write down the exact code, and what the appliance was doing when it appeared.
- Check whether it's still under warranty. If it is, the repair may cost you nothing.
Quick answers
What does the Ideal F4 fault code mean?
There's a fault with the flow thermistor, a temperature sensor inside the boiler. Ideal says it plays an essential role in keeping the boiler at the right temperature.
Can I fix Ideal F4 myself?
Engineer only (about 5 minutes to reset). Start with: reset the boiler. If the code comes back, contact Ideal quoting F4 (or L4): it needs an engineer visit. Outside the warranty, use a Gas Safe registered engineer.
Is Ideal F4 worth repairing?
Worth fixing. A thermistor is a small part, but it's inside the boiler, so an engineer fits it.
Checked against
- Ideal Heating: what does my boiler fault code mean
- Ideal Heating: Logic Combi 24, 30 and 35 troubleshooting guide
- Ideal Heating: boiler fault codes, page 2
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.