Saunier Duval F.42 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.42 fault code mean?
The F.42 code (sometimes displayed as F42, with the decimal point appearing between flashing digits on the display) signals one of two related electronic faults depending on your Saunier Duval model. On most models it indicates a heating sensor fault — specifically a problem with an NTC thermistor that monitors flow or return water temperature. On newer condensing models such as the Thema Condens F 25 it can instead point to a coding resistor or eBus communication fault, meaning the main PCB cannot correctly identify the boiler's configuration or has lost reliable communication with a connected controller. In practice both interpretations sit within the same family of electronic and sensor-circuit faults, and a Gas Safe engineer will quickly establish which applies to your specific unit.
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
- Disconnected or poorly seated NTC sensor Common
The NTC thermistor that measures heating flow or return water temperature has worked loose from its clip on the pipe. When the sensor loses firm contact with the pipework, its readings become erratic or drop out entirely, triggering F.42. This is the most straightforward cause and the first thing an engineer will physically check.
- Faulty NTC sensor cable or connector Common
Even if the sensor body itself is seated correctly, a damaged cable or a corroded plug connection between the sensor and the PCB can interrupt the signal. Vibration, heat cycling, and age all degrade wiring over time, and a high-resistance or open-circuit connection produces the same fault as a completely absent sensor.
- Corroded eBus wiring or connectors Sometimes
On condensing models where F.42 relates to eBus communication, oxidation builds up on the connector pins over time and raises contact resistance enough to disrupt signal transmission. The fault often starts appearing intermittently before becoming permanent as the corrosion progresses — a useful clue when diagnosing.
- eBus device conflict or overload Sometimes
If more than one external controller, thermostat, or programmer is connected to the boiler's eBus terminals, a faulty or incompatible device can overload or short the bus, preventing normal communication. Disconnecting devices one by one can isolate the culprit.
- Failed NTC thermistor Sometimes
The sensor element itself has reached the end of its working life or suffered damage from a system contamination event. A failed thermistor typically reads either open-circuit or far outside its expected resistance range and must be replaced rather than cleaned or reseated.
- Faulty main PCB Rare
If the coding resistor, eBus wiring, and all sensors check out as good, the PCB's own circuitry — potentially damaged by a power surge, repeated thermal stress, or ageing capacitors — may be misreading the sensor or coding resistor input. This is the least common cause but the most expensive to remedy.
How to fix it
- Check for an obvious cause before calling anyone DIY safe
Look at the boiler display to confirm F.42 is showing. Note whether it appeared suddenly or has been intermittent — this helps the engineer narrow down whether it is a sensor issue or an eBus communication problem. Check that your gas supply is on (other gas appliances should work normally) and that your system pressure gauge reads between 1 and 1.5 bar. If pressure is low, top up via the filling loop until it reaches about 1.2 bar, then attempt a single reset by pressing the boiler's reset button.
- Attempt a boiler reset DIY safe
Press and hold the reset button (usually marked with a flame and a cross, or labelled 'Reset') for a few seconds until the boiler attempts to restart. If it fires up and runs normally, monitor it closely over the next 24 hours. If F.42 returns within a short time, do not keep resetting — persistent faults require investigation, and repeated resets without fixing the root cause can mask a developing problem.
- Check whether an external controller or thermostat was recently added or changed DIY safe
If F.42 appeared shortly after a new room thermostat, programmer, or smart controller was connected to the eBus terminals, try disconnecting that device and resetting the boiler. If the fault clears, the added device is likely incompatible or incorrectly wired — inform your engineer before they visit.
- Have a Gas Safe engineer inspect and reseat the NTC sensor Gas Safe engineer
An engineer will locate the heating flow and return NTC thermistors, check that each is properly clipped to its pipe, and measure the sensor's resistance with a multimeter to confirm it falls within the manufacturer's specified range for the current water temperature. If a sensor has worked loose, reseating it and clearing the fault code may be all that is needed.
- Inspect wiring, plugs, and eBus connectors for corrosion or damage Gas Safe engineer
The engineer will check the cables running from each NTC sensor to the PCB for chafing, heat damage, or corroded plug terminals. On condensing models suspected of an eBus fault, the eBus wiring and all connected device plugs will be tested for continuity and contact resistance. Corroded connectors are cleaned or replaced as needed.
- Replace the NTC thermistor if it has failed Gas Safe engineer
If the sensor itself measures outside its acceptable resistance range, the engineer will fit a replacement NTC thermistor. These are low-cost parts and the work is normally completed within a single visit. Bear in mind that Saunier Duval is no longer actively marketed in the UK, so the engineer may need to source the part from a specialist supplier — it is worth asking them to confirm parts availability before the appointment.
- Replace the main PCB if all other components test satisfactory Gas Safe engineer
Should the sensor, wiring, and eBus all prove to be in good order, the PCB itself is the likely culprit. PCB replacement is a more involved and expensive repair. Given Saunier Duval's limited UK parts availability, ask the engineer to obtain a firm quote for the part before committing — in some cases, a boiler of this age may be more economically replaced than repaired with a new PCB.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If the reset did not clear the fault, or if F.42 has returned after resetting, contact a Gas Safe registered engineer. You can verify any engineer's registration at GasSafeRegister.co.uk. All work on the boiler's internal components is a legal requirement to be carried out by a Gas Safe registered professional in the UK.
Parts you may need
- NTC thermistor (flow or return sensor) · from £18
- Sensor cable and connector harness · from £25
- eBus connector/wiring loom · from £30
- Main PCB (control board) · from £280
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–£300, depending on the underlying cause.
Frequently asked questions
Is F.42 the same as F42 on my Saunier Duval boiler?
Yes, they are identical. The display alternates two numbers and uses a decimal point as part of how it shows fault codes, so F.42 and F42 refer to exactly the same fault. You may see it written either way in documentation or online, but the diagnosis and fix are the same.
Can I fix F.42 myself without calling an engineer?
Your DIY options are limited but worth trying first: check your system pressure is between 1 and 1.5 bar and top it up via the filling loop if needed, then attempt one or two resets. If you recently connected a new eBus controller or thermostat, try disconnecting it and resetting. Beyond that, the fault involves internal sensors, wiring, and potentially the PCB — all of which must be inspected and repaired by a Gas Safe registered engineer under UK law.
Why does my Saunier Duval show F.42 only sometimes and then clear itself?
Intermittent F.42 faults are a strong indicator of a connection or corrosion problem rather than a completely failed component. Corroded eBus connector pins or a sensor cable with a partial break can lose contact under certain conditions (vibration, temperature changes) and then temporarily restore it. Left unattended, intermittent faults of this type tend to become permanent as corrosion or wear progresses, so it is worth booking an engineer rather than waiting.
How much does it cost to repair F.42 on a Saunier Duval?
Most people pay between £100 and £300 for this repair. A simple sensor reseating or connector clean is at the lower end of that range once the call-out and labour are included; replacing an NTC thermistor typically comes to £100–£200 in total. A PCB replacement is significantly more expensive — parts alone can cost £200–£350, pushing the total bill well above £400. Given that Saunier Duval is no longer widely sold in the UK, parts can sometimes be harder to source, so ask your engineer to confirm availability and pricing before they start work.