Ferroli F42 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli F42 fault code mean?
The F42 code appears when the Ferroli boiler's control board detects an abnormal or implausible reading from one or more of its NTC thermistors — specifically the heating flow sensor or the central heating return sensor. On many Ferroli models the fault triggers when the difference between the flow and return sensor readings exceeds around 12°C, suggesting at least one sensor is producing an inaccurate value. Because these sensors feed directly into the boiler's safety protection logic, the board locks the boiler out rather than risk overheating. You may also see this displayed as A42 on certain Ferroli models — both relate to the same underlying sensor protection fault.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or degraded NTC flow or return sensor Common
Over time, NTC thermistors drift out of their correct resistance range or develop an internal open circuit, causing the PCB to read an implausible temperature. Because F42 is triggered by a discrepancy between the two sensors, even one failing thermistor is enough to lock the boiler out. This is by far the most common root cause.
- Damaged or disconnected sensor wiring Common
The wiring loom connecting the flow or return sensor to the PCB can develop a short, a break, or a loose plug — especially in older boilers where wiring insulation has become brittle. A disconnected sensor plug will produce an out-of-range reading identical to a faulty sensor itself.
- Air pocket or low circulation causing a genuine temperature differential Sometimes
If water circulation through the heat exchanger is poor — due to a failing pump, a partially closed valve, or a significant air lock — the actual temperature difference between flow and return may genuinely exceed the protection threshold, triggering F42 as intended rather than as a false alarm.
- PCB fault Rare
In rare cases, the control board itself develops a fault that causes it to misinterpret correct sensor signals, or a previous sensor failure has damaged the relevant input circuit on the PCB. This is a last-resort diagnosis after sensors and wiring have been ruled out.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button — usually marked with an 'R' or a flame symbol on the front panel — and press or hold it for around 5 seconds as described in your user manual. If the boiler fires up and runs normally, monitor it closely over the next hour. A single one-off lockout can occasionally occur without a deeper fault.
- Do not keep resetting if the code returns DIY safe
If F42 reappears after one or two resets, stop resetting. Repeatedly clearing a genuine sensor fault without fixing the underlying problem can mask the issue and, in a worst case, stress other components. Three lockouts in a short period confirms a real fault that needs an engineer.
- Check your gas supply and system pressure DIY safe
Before calling an engineer, quickly confirm the obvious basics: make sure other gas appliances in your home are working (ruling out a supply interruption), and check the pressure gauge on your boiler. It should read between 1.0 and 1.5 bar. If it has dropped below 1.0 bar, top it up via the filling loop following your boiler's user manual. Low pressure alone won't cause F42, but it is worth noting before the engineer arrives.
- Have a Gas Safe engineer inspect the sensor wiring and connections Gas Safe engineer
An engineer should gain access to the boiler's internals and visually inspect the wiring loom to both the flow and return NTC sensors. Loose plugs, corroded terminals, chafed cable insulation, and pinched wires are all common culprits that can be identified and rectified quickly at this stage.
- Test both NTC thermistors and replace any faulty ones Gas Safe engineer
Using a multimeter, the engineer will measure the resistance of each sensor at a known water temperature and compare the reading against Ferroli's resistance table. A sensor reading significantly outside the expected range should be replaced. Given that sensors are relatively inexpensive, many engineers replace both flow and return sensors together if either is suspect.
- Assess pump and circulation if sensors test healthy Gas Safe engineer
If both sensors and their wiring check out correctly, the engineer should assess whether a genuine temperature differential is occurring — checking the central heating pump for correct speed and output, looking for air locks in the system, and verifying that all isolation valves are fully open.
- Consider PCB replacement as a last resort Gas Safe engineer
If all sensors, wiring, and circulation checks pass, the control board itself may be misreading correct signals. PCB diagnosis and replacement must be carried out by a Gas Safe registered engineer. On older Ferroli models, parts availability should be confirmed before committing to this repair.
- Call a Gas Safe registered engineer Gas Safe engineer
All internal boiler work — sensor testing, wiring repairs, component replacement, and PCB work — must be carried out by a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk before booking.
Parts you may need
- NTC flow thermistor (Ferroli) · from £18
- NTC return thermistor (Ferroli) · from £18
- Sensor wiring loom / harness · from £35
- Central heating pump · from £75
- Ferroli PCB / control board · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£280, depending on the underlying cause.
Frequently asked questions
Is Ferroli F42 dangerous — do I need to turn the boiler off?
F42 is a safety lockout, which means the boiler has already shut itself down as a precaution. You do not need to isolate the boiler manually, but you should not repeatedly reset it without getting the fault diagnosed. Leave it locked out, keep warm with alternative heating if needed, and arrange for a Gas Safe engineer to inspect it promptly.
Can I fix F42 myself, or do I need an engineer?
A homeowner can safely attempt a single reset and carry out basic checks such as confirming the gas supply and system pressure. However, everything beyond that — testing sensors with a multimeter, replacing thermistors, repairing wiring, or touching the PCB — must be done by a Gas Safe registered engineer. These tasks involve working inside the boiler casing on electrical and potentially gas-adjacent components.
How much will it cost to fix Ferroli F42 in the UK?
For most people, F42 turns out to be a faulty NTC sensor or wiring issue. Expect to pay roughly £100–£180 for a call-out, diagnosis, and a new sensor fitted. If both sensors need replacing the cost is similar given parts are inexpensive. If the PCB is found to be at fault after everything else has been ruled out, costs typically rise to around £250–£280 including parts and labour. On an older Ferroli, always ask the engineer to confirm parts are available before proceeding, as discontinued models can have longer lead times.
Why does Ferroli F42 come back after resetting?
A reset clears the lockout flag on the PCB but does nothing to fix the underlying fault. If the flow or return sensor is genuinely producing an out-of-range resistance value, or its wiring is intermittently shorting, the boiler will detect the problem again as soon as it fires up and lock out once more. Recurring F42 codes almost always mean a physical component needs attention — don't keep resetting and hope it clears on its own.