F.42
Vaillant Boiler
Severity: ModerateWhat Does This Error Mean?
Vaillant F.42 means the coding resistor circuit is faulty.
The coding resistor tells the PCB the boiler model — output rating, gas type, and configuration.
Without a valid reading the PCB will not run.
Cause is almost always a recent PCB swap with the wrong coding resistor or one that came loose, less commonly a damaged wire on the resistor harness.
Affected Models
- Vaillant ecoTEC Plus
- Vaillant ecoTEC Pro
- Vaillant ecoFIT Pure
- Vaillant ecoFIT Sustain
Common Causes
- Recent PCB swap without transferring the coding resistor
- Coding resistor connector loose after service work
- Wrong coding resistor for the boiler model
- Damaged wire on the resistor harness
- Resistor itself failed (rare)
- PCB resistor input damaged
How to Fix It
-
Confirm any recent service.
F.42 most commonly appears right after PCB replacement, gas valve swap, or any other major service.
If the boiler was working before service and posted F.42 immediately after, the engineer who did the work needs to come back. -
Do not attempt DIY.
F.42 work is inside the PCB area on a gas appliance — Gas Safe certification required by UK law.
Any DIY attempt beyond a reset button voids insurance and can invalidate the boiler warranty. -
Reset once.
Press the reset button on the front of the boiler.
If F.42 was a transient glitch from a power dip during boot, reset clears it.
If F.42 returns immediately, call an engineer. -
Engineer step: locate the coding resistor.
The coding resistor sits on a small connector on the PCB (typically labelled X20 or similar on the wiring diagram).
Visual inspection: a small resistor body with two leads going into a connector. -
Engineer step: confirm correct part.
Coding resistors come in different values per model.
The Vaillant service manual lists the correct resistor for each boiler.
A wrong resistor will be detected on every boot — F.42 returns immediately. -
Engineer step: reseat or replace.
If the resistor is correct, reseat the connector firmly.
If wrong, fit the right one (1-5 GBP part).
Boiler should boot to normal operation. -
Final test.
After repair, run a heating call and a hot water call.
Both should fire normally.
F.42 returning under load is rare — if it happens, the PCB itself is failing and needs replacement.
Frequently Asked Questions
Why does Vaillant use coding resistors at all?
One PCB part number covers many boiler models — the coding resistor tells the firmware which boiler it is in.
This reduces parts inventory.
The trade-off is that a swapped PCB needs the resistor moved over, and an engineer who forgets posts F.42 immediately.