Boiler fault codes
Ideal F4 Fault Code: Flow Thermistor Fault
Published
Ideal display
F4
What it means
On an Ideal boiler, F4 (L4 on some control boards) is a flow thermistor fault: the board is getting a reading from the flow temperature sensor that makes no sense, so it stops the boiler.
Safe to check yourself
- Confirm the badge says Ideal, because F4 means something different on other makes
- Check the pressure gauge reads 1 to 1.5 bar with the system cold
- Make sure the valves under the boiler and on the radiators are open
- Press restart once
Call an engineer if
- F4 or L4 returns after one restart
- The boiler makes banging or kettling noises before the code appears
- Pipes near the boiler are much hotter than usual
- You smell gas: open windows, turn off at the meter if safe, avoid anything that sparks, then leave and call the National Gas Emergency Service on 0800 111 999
An F4 fault on an Ideal boiler is a flow thermistor fault. The flow thermistor is the temperature sensor at the top of the heat exchanger, and the control board has stopped trusting its reading. Check the pressure is 1 to 1.5 bar and the valves are open, restart once, and call an engineer if F4 comes back.
What the flow thermistor does
The flow thermistor measures the temperature of the water leaving the heat exchanger. The board uses it to control the burner and to stop the boiler overheating. If the reading goes out of range, because the sensor has failed, a wire has come loose or the reading is implausible, the board stops the boiler rather than run blind.
On an Ideal Logic Combi, Ideal lists the code as F4 or L4 depending on the board, both meaning the same thing. A related code, F5 or L5, is the return thermistor on the pipe coming back from the radiators.
Sensor fault or circulation problem? Sources disagree
Ideal's installation manual treats F4 as an electrical fault in the sensor circuit. Its fault-finding chart checks the wiring at the thermistor, measures the sensor's resistance, then checks the connector on the control board. Nothing in that chart is about water flow.
Many heating firms add that F4 is often linked to poor circulation: a sticking pump, closed valves, sludge or low pressure making the water in the heat exchanger overheat. Both can be true on the day. That is why the pressure and the valves are worth checking before you call, and why a good engineer checks the pump as well as the sensor.
How an engineer tests an F4
The Logic manual gives exact resistance values for the thermistor, so the test is quick and conclusive. With the wires off, the sensor should read about 10,000 ohms at 25°C, 2,500 at 60°C and 1,000 to 1,100 at 85°C. If it is out of range, it is replaced. If it is in range, the engineer checks the wiring to the board's low-voltage connector, and only then the board.
- Loose or corroded plug on the thermistor: re-made at no parts cost
- Failed thermistor: one of the cheapest parts in the boiler
- Damaged harness: repaired or replaced
- Control board: the dearest outcome, and the last one after the cheaper checks
The thermistor screws into the heat exchanger, and Ideal's manual has the boiler drained down, the new sensor fitted with a fresh sealing washer and the boiler refilled. That is inside the casing and is not a DIY job.
Getting it fixed
You pay the £60 call-out to get an engineer out, and the repair price is agreed with you on site before any work starts. Gas work is carried out by Gas Safe registered engineers. Because the thermistor is a common part, the fix is often done on the first visit.
If the boiler is also noisy or the radiators heat unevenly, sludge may be part of the story; see our guide to boiler pump noise. To book, use Ideal boiler repair. Seeing F4 on a Viessmann instead? Go to the Viessmann F4 page, which is a different fault.
Questions people ask
Can I keep using an Ideal boiler with an F4 fault?
No. The boiler locks itself out because it cannot measure its own water temperature. If one restart clears it and it stays clear, keep an eye on it. If it returns, leave it off and book a repair.
Is F4 the same as L4 on an Ideal?
Yes. Ideal's Logic Combi manual lists them together as the flow thermistor fault. Which one you see depends on the control board fitted.
Can low pressure cause an F4?
Low pressure normally shows as F1 on an Ideal. But poor circulation from low water, air or a sticking pump can make the flow temperature behave oddly, which is why checking the pressure first is worthwhile.