Vaillant F.42 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.42 fault code mean?
The F.42 code appears when your Vaillant boiler cannot correctly read its coding resistor — a small component (sometimes a plug or inline connector) that identifies the boiler model variant and the type of gas it runs on to the PCB. When the PCB detects a short circuit, an open circuit, or a mismatch between the physical resistor and the gas-type setting stored in diagnostic parameter D.087, it locks the boiler out as a safety measure. In plain terms, the boiler has lost confidence in its own identity and refuses to fire until the issue is resolved. This is entirely an engineer-level fault; there are no meaningful homeowner checks that will clear it.
General guidance only — not a substitute for professional advice. 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
- Coding plug not seated correctly in PCB socket X24 Common
Vaillant ecoTEC PCBs use a small colour-coded plug inserted into socket X24 to identify the gas type: yellow for natural gas, grey for LPG. If this plug has worked loose, was never fully pushed home, or has been dislodged during previous work, the PCB cannot read it and triggers F.42. This is the most straightforward cause and is often resolved quickly by a Gas Safe engineer.
- Wiring harness fault at the coding resistor Common
On some Vaillant models the coding resistor is a small white inline connector within the wiring loom rather than a plug on the PCB. Corrosion, a damaged connector, or a chafed wire between this resistor and the PCB can create the open-circuit or short-circuit condition the F.42 code describes. An engineer will inspect and test the harness with a multimeter.
- Gas-type parameter D.087 set incorrectly Sometimes
Vaillant boilers store the gas type in diagnostic parameter D.087. If a PCB has been replaced and this parameter was not configured to match the physical coding plug — or if the wrong plug was fitted — the boiler detects a mismatch and locks out with F.42. This is particularly common after a PCB swap where the Device Specific Number (DSN) was also not adjusted.
- Faulty or damaged PCB Sometimes
If the PCB's input circuit for the coding resistor has been damaged — by a power surge, moisture ingress, or component failure — it may be unable to read a perfectly good coding plug. In this scenario the board itself needs replacing. Vaillant ecoTEC boards are known to fail in service, and F.42 can be one of the resulting codes. A concurrent F.70 code (DSN/PCB communication fault) alongside F.42 is a strong indicator of a board-level problem.
- Wrong or missing coding resistor for the boiler variant Rare
Each Vaillant model variant requires a specific coding resistor or plug part number. If a previous repair used the wrong part, or the resistor is missing entirely, the boiler will display F.42 on every startup. This is relatively uncommon but can occur after non-specialist repairs.
How to fix it
- Do not attempt to reset the boiler repeatedly DIY safe
A single reset attempt is reasonable, but because F.42 is a hardware identification fault rather than a transient error, repeated resets will not resolve it and may mask useful diagnostic information for the engineer. If the boiler does not stay running after one reset, leave it and call for help.
- Check whether an F.70 code is also displayed DIY safe
Note down every code shown on the display before the engineer arrives. If F.70 appears alongside or alternates with F.42, mention this to the engineer — it points toward a PCB or DSN issue and helps them prepare the right parts.
- Engineer: inspect coding plug at PCB socket X24 Gas Safe engineer
A Gas Safe engineer should check that the colour-coded coding plug (yellow for natural gas, grey for LPG) is the correct part for the boiler model and is fully seated in socket X24. Re-seating or replacing the plug is often all that is needed.
- Engineer: test the wiring harness and inline coding resistor Gas Safe engineer
Using a multimeter, the engineer should check continuity and resistance values through the wiring loom and any inline coding resistor connector. Any open circuit, short, or out-of-range resistance reading indicates a harness fault requiring repair or replacement.
- Engineer: verify and set diagnostic parameter D.087 Gas Safe engineer
The engineer should access the service menu and confirm that D.087 is set to the correct gas type for the installation. A mismatch between this parameter and the physical coding plug is a common cause of F.42 following PCB replacement.
- Engineer: adjust the Device Specific Number (DSN) if a new PCB has been fitted Gas Safe engineer
When a Vaillant ecoTEC PCB is replaced, the replacement board is a generic unit shared across the ecoTEC range. The engineer must configure the DSN parameter to pair the board correctly with the specific boiler model; failure to do so can produce F.42 and F.70 simultaneously.
- Engineer: replace the PCB if it is confirmed faulty Gas Safe engineer
If all connections, the coding plug, and the parameter settings are correct yet F.42 persists, the PCB's own input circuitry is likely damaged. The engineer will source the correct replacement board, fit it, and configure the DSN and gas-type parameters before testing.
- Call a Gas Safe registered engineer if you have not already done so DIY safe
Every repair step beyond noting the fault code and attempting a single reset requires a Gas Safe registered engineer. Contact a local engineer or Vaillant's service line on 0330 100 3143 — if your boiler is under warranty, the repair may be covered at no cost.
Parts you may need
- Vaillant coding plug (natural gas, yellow — X24) · from £18
- Vaillant coding plug (LPG, grey — X24) · from £18
- Vaillant wiring harness (model-specific) · from £65
- Vaillant ecoTEC PCB (replacement, model-specific) · from £220
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 £80–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Vaillant F.42 fault myself?
No. Unlike some Vaillant faults where a homeowner can top up pressure or thaw a condensate pipe, F.42 requires hands-on work with the PCB, wiring harness, and boiler parameters. All of these tasks must be carried out by a Gas Safe registered engineer. A single reset attempt is fine, but beyond that you should call a professional.
How much does it cost to fix a Vaillant F.42 fault?
If the fault turns out to be nothing more than a loose or incorrect coding plug, an engineer visit including labour to diagnose and re-seat or replace the plug will typically come to around £80–£150. If the wiring harness needs repair or replacement, expect £150–£250. A full PCB replacement — parts and labour — usually falls in the £300–£450 range, which is why it is worth getting a proper diagnosis before agreeing to a board swap. Very occasionally a persistent F.42 on an older boiler can make replacement more economical than repair; a new boiler installation typically starts around £2,000.
Why does F.42 appear after a PCB has been replaced on my Vaillant ecoTEC?
Vaillant ecoTEC replacement PCBs are generic units designed to work across the entire ecoTEC range. To function correctly in a specific model, the engineer must configure two things after fitting: the Device Specific Number (DSN), which pairs the board to the boiler variant, and the gas-type parameter D.087, which must match the physical coding plug in socket X24. If either of these steps is skipped, F.42 (and often F.70) will appear immediately. An experienced Vaillant engineer will be aware of this requirement.
What is the difference between F.42 and F.70 on a Vaillant boiler?
F.42 specifically relates to the coding resistor — the component that identifies the gas type and model variant to the PCB. F.70 is a DSN (Device Specific Number) or PCB communication error, indicating the board cannot identify the boiler it is installed in. The two faults are closely related and frequently appear together, particularly after a PCB replacement where the DSN has not been set. An engineer seeing both codes will focus on the PCB configuration and coding plug as a combined issue rather than treating them as two separate faults.