Protherm F.42 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.42 fault code mean?
The F.42 code appears on Protherm boilers when the control board detects that the coding resistor (R1) is returning a value outside the expected range. The coding resistor is a small component on the PCB that effectively tells the boiler's electronics what type of appliance it is and how it should behave. When this value doesn't match what the board expects — whether because the resistor has failed, the wrong product code was programmed in at installation, or the PCB itself is damaged — the boiler locks out and displays F.42 as a safety precaution. This is a board-level electronic fault and sits in the more serious category of Protherm error codes (F.20–F.42), meaning it will not clear on its own without the underlying issue being addressed.
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
- Failed or out-of-range coding resistor (R1) Common
The coding resistor R1 sits on the control board and defines the boiler's identity to its own electronics. Over time, through general wear, heat cycling, or manufacturing defects, the resistor can drift outside its specified resistance value or fail completely. This is the most frequent root cause of F.42 on Protherm boilers.
- Incorrect product code entered during commissioning Sometimes
When a Protherm boiler is first installed or a PCB is replaced, an engineer must enter the correct product code in the installation menu. If the wrong code is entered, the boiler may display F.42 — sometimes alternating with an F.70 code. This is a programming error rather than a hardware failure and is straightforward to correct once identified.
- Damaged or faulty PCB (control board) Sometimes
Power surges, prolonged overheating, or moisture ingress can damage the printed circuit board itself. If the board is unable to read or communicate with the coding resistor correctly — even if the resistor is physically intact — an F.42 fault will be triggered. A damaged PCB may show other accompanying faults or erratic behaviour.
How to fix it
- Do not keep resetting the boiler repeatedly DIY safe
Unlike some simple lockouts, F.42 is an electronic fault that will not resolve itself with repeated resets. One reset attempt is fine to confirm the fault persists, but cycling the power repeatedly risks stressing other components on the board. If the code returns immediately after a reset, leave the boiler off and move to the next steps.
- Check your gas supply is working DIY safe
Before calling an engineer, quickly confirm that other gas appliances in your home (such as a gas hob) are functioning normally. This rules out a wider gas supply interruption as a contributing factor, though it is unlikely to be the direct cause of F.42.
- Note whether F.42 is alternating with F.70 DIY safe
If your display is cycling between F.42 and F.70, make a note of this and tell your engineer. This pattern strongly suggests the issue is an incorrectly programmed product code rather than a hardware failure, which is a quicker and cheaper fix.
- Have a Gas Safe engineer measure the resistance of R1 Gas Safe engineer
An engineer will isolate the boiler electrically, access the control board, and use a calibrated multimeter to measure the resistance of R1. The reading is compared against the value specified in the Protherm technical documentation for your specific model. If the resistor is out of tolerance, it can be replaced without necessarily changing the entire PCB.
- Ask the engineer to verify the programmed product code Gas Safe engineer
If R1 measures within specification, the engineer should check the installation menu to confirm the correct product code is entered for your exact boiler model and output rating. Correcting a misconfigured code is a simple fix that avoids unnecessary component replacement.
- Replace the PCB if R1 and the product code are both correct Gas Safe engineer
If the coding resistor tests correctly and the product code is accurate, the fault lies within the control board itself. A Gas Safe engineer will source and fit a compatible replacement PCB for your Protherm model, re-programme it correctly, and verify the boiler fires and operates safely before leaving.
- Call a Gas Safe registered engineer Gas Safe engineer
All of the diagnostic and repair work for F.42 involves the boiler's electrical control board and must be carried out by a qualified Gas Safe registered engineer. You can verify an engineer's registration at the Gas Safe Register website (gassaferegister.co.uk). Ask them to diagnose whether only R1 needs attention or whether a full PCB swap is required, as this significantly affects the cost.
Parts you may need
- Coding resistor R1 · from £15
- Protherm replacement PCB (control board) · 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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I fix a Protherm F.42 fault myself?
No — this is not a homeowner repair. The F.42 fault is rooted in the boiler's electronic control board, and any work on PCB components must be carried out by a Gas Safe registered engineer. There are no homeowner-safe checks that will resolve this fault beyond a single reset attempt.
How much does it typically cost to repair a Protherm F.42 fault?
Most people pay somewhere between £150 and £380 all-in, depending on whether the coding resistor alone needs replacing (cheaper, around £150–£200 including labour and call-out) or whether the entire PCB needs swapping (typically £250–£380 for parts and labour). If your boiler requires a particularly hard-to-source board, costs can occasionally climb higher — worth factoring in if the boiler is over ten years old.
Why is F.42 sometimes showing alongside F.70 on my Protherm boiler?
If your display alternates between F.42 and F.70, this is a strong indicator that the product code entered in the boiler's installation menu does not match the actual model. This is a programming issue rather than a hardware failure and is typically one of the quicker fixes — your engineer simply needs to access the installation settings and enter the correct code for your specific Protherm appliance.
Is it worth repairing a Protherm boiler showing F.42, or should I replace it?
If your boiler is under ten years old and otherwise in good condition, repair is almost always the more cost-effective route. If it is older than ten to twelve years, already needed several repairs recently, or spare parts are difficult to source, it may be worth getting a quote for a new boiler alongside the repair estimate. A Gas Safe engineer can give you an honest view of the boiler's overall condition during the diagnostic visit.