Vaillant F.42 fault codewhat it means and how to fix it
A fault with the coding resistor, the small component that tells the board which gas the boiler is set up for. The boiler will not run until it knows.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.42
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.42 means
Vaillant's ecoTEC boilers are built for natural gas or LPG and the board learns which from a coding resistor in the cable harness. F.42 is that resistor reading wrong. The ecoTEC plus table says "coding resistor or gas group resistor causes a short circuit" and has the engineer check the plug, the earth connection, the cable and the resistor itself, and the ecoTEC exclusive adds the other failures: an interruption rather than a short, the resistor missing altogether, and the resistor not matching the gas type selected under diagnostic code D.087. That last cause is why F.42 clusters around conversions and board changes.
A boiler converted to LPG needs the LPG resistor and the matching D.087 setting; fit one without the other and the board refuses to fire on a gas it is not sure of, which is exactly what you want it to do.
On the older turboMAX plus the same number is "faulty cable loom — faulty or incorrect loom fitted", the resistor living in the loom there too. The 2022 CS/1-5 keeps the code as F.042, "the coding resistor (in the wiring harness) or the gas group resistor (on the PCB, if available) is invalid", and its check list is all harness: an interruption in the wiring to the fan, the wrong harness fitted between the PCB and the gas valve (checked by article number), and the heat cell coding resistor not detected at PCB plug X25. The ecoTEC pro, ecoFIT and ecoTEC sustain tables do not carry F.42; on those boilers the gas type check is reported differently.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Coding resistor missing or wrong after a conversion | The exclusive table's "coding resistor missing" and "does not match the gas type selection under D.087". A natural gas to LPG conversion, or a harness change, done without the matching resistor. | Common |
| Short circuit or break in the resistor circuit | Vaillant's ecoTEC plus wording. The resistor sits in the harness where a chafed lead or a damp plug can short it, and a pulled lead can open it. | Common |
| Earth connection fault | Named on the ecoTEC plus check list. A poor earth changes what the board measures across the resistor. | Occasional |
| Faulty or incorrect loom on a turboMAX plus | The legacy meaning: the whole cable loom is the wrong one for the appliance or has failed. A parts question rather than a setting. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£200
Time on site
45 minutes
A resistor or plug repair is £90 to £150. A cable harness is £120 to £200 fitted. If a recent conversion caused it, the fault is the converting firm's to put right.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant main PCB (electronics box)
£160–£340Authorised Vaillant parts distributor
Price checked 28 September 2026
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.62Stop — call an engineerThe gas valve is not shutting off when the boiler tells it to. This is a gas safety fault and the boiler must not be used until an engineer has seen it.Related: gas valve and supply
- F.61Get it checkedThe gas valve control circuit has faulted. The boiler will not fire and this is firmly an engineer's job.Related: gas valve and supply
- F.53Get it checkedThe gas is not flowing through the valve the way the boiler expects. Vaillant's list runs from low gas pressure to a wet or blocked filter under the venturi cap.Related: gas valve and supply
- F.54Get it checkedA gas supply fault, usually shown alongside F.28 or F.29. It means gas is reaching the appliance at too low a pressure.Related: gas valve and supply
- F.26Get it checkedVaillant call this "fuel valve not working": the stepper motor that drives the gas valve is not responding, so the boiler cannot open the gas and will not light.Related: gas valve and supply
- F.88Get it checkedThe board cannot see the gas valve. Vaillant print F.88 as the valve not connected or connected incorrectly, or a harness fault, and the boiler will not run until the circuit is proved.Related: gas valve and supply
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.42
The boiler has just been converted to LPG. Is F.42 expected?
It is the classic trigger. Vaillant's conversion needs the LPG coding resistor and the D.087 gas type to agree, and the code appears when they do not. The converting engineer should return to finish the job.
Is the boiler safe to leave switched on?
Yes. F.42 holds the burner off precisely because the board cannot confirm the gas type. Leaving the boiler powered keeps the display and frost protection working.
Why is the coding resistor not in my ecoTEC pro manual?
The pro, the ecoFIT and the ecoTEC sustain tables have no F.42 row. F.42 is a code of the ecoTEC plus ..6/5-5, the exclusive, the 2022 CS/1-5 as F.042 and, in its cable-loom form, the turboMAX plus.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.