Protherm F.43 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.43 fault code mean?
The F.43 fault code appears on Protherm boilers when the control board is unable to identify or confirm which boiler variant it is running. In other words, the PCB cannot match the boiler's identity to an expected value, so it locks the unit out as a safety precaution. On the display, the letter 'F' will flash alongside '43'. This code sits in the same fault family as F.42 — which flags a problem with the coding resistor itself — and the two codes share several root causes. F.43 specifically means the variant value read by the board is wrong or unrecognised, rather than simply that the resistor is absent or open-circuit.
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
- Faulty or out-of-tolerance coding resistor (R1) Common
The coding resistor is a small component on the control board that tells the PCB exactly which boiler variant it is installed in. If this resistor drifts out of its expected value, fails due to wear, or has suffered mechanical damage, the board receives an unrecognised reading and triggers F.43. This is the single most common root cause of this code.
- Incorrect boiler variant code in the installer menu Sometimes
During installation or after a PCB swap, an engineer must enter the correct product code in the installer menu. If the wrong code was entered — or the setting was lost after a power interruption — the board will report an incorrect variant value, sometimes alternating with an F.70 code on the display.
- Faulty or damaged PCB Sometimes
A control board that has been damaged by a voltage surge, overheating, or internal component failure may misread or be unable to process the coding resistor signal at all, producing F.43 even when the resistor itself is fine. PCB faults are less common but represent the costlier outcome.
- Unstable mains electrical supply Rare
Significant voltage spikes or dips in the household electrical supply can cause the boiler's electronics to behave erratically, tripping fault codes including F.43. If the code appears suddenly and clears after a reset with no recurrence, a transient supply issue may be to blame.
How to fix it
- Reset the boiler DIY safe
Switch the boiler off at the power switch or isolate it at the fused spur, wait around 30 seconds, then switch it back on. A single transient electronic glitch can sometimes cause F.43, and a cold restart may clear it. If the code returns immediately or after a second reset attempt, do not keep resetting — move on to the next steps.
- Check your system pressure DIY safe
Look at the pressure gauge on the front of the boiler. Protherm boilers typically require between 1.0 and 1.5 bar when cold. If the needle sits below 1 bar, top up via the filling loop until the pressure is in range, then attempt a reset. Although low pressure alone does not cause F.43, it is always worth ruling out before calling an engineer.
- Check that your gas supply is live DIY safe
Confirm that other gas appliances in the property (hob, gas fire) are working normally. If there is no gas supply, contact your gas supplier rather than an engineer. If the supply is fine, the issue lies within the boiler itself.
- Have an engineer check and test the coding resistor (R1) Gas Safe engineer
A Gas Safe registered engineer should open the boiler casing and locate the coding resistor on the control board. Using a multimeter, they can verify whether R1 is reading the correct resistance value for the boiler variant. If the resistor has failed or drifted, replacing this small component is typically the most cost-effective fix.
- Have an engineer verify the installer menu boiler variant setting Gas Safe engineer
The engineer should access the installer menu and confirm that the correct product code for the specific boiler variant is entered. An incorrect or reset setting here will cause a persistent F.43 and must be corrected before the boiler will run normally.
- Have the wiring and board connections inspected Gas Safe engineer
The engineer should check all wiring harnesses and connectors associated with the control board for signs of corrosion, loose pins, or chafing. A poor connection to the coding resistor circuit can mimic a component failure without the component itself being at fault.
- Have the PCB diagnosed and replaced if necessary Gas Safe engineer
If the coding resistor tests correctly and all settings and wiring are in order, the control board itself is likely faulty. The engineer will confirm this by diagnosis and, if needed, fit a replacement PCB correctly coded for the boiler variant. Given the cost of a new board, the engineer may also advise whether repair is economically worthwhile on an older boiler.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
F.43 is fundamentally an electronic control fault that requires specialist diagnosis. If the boiler has not cleared after a reset and you have confirmed pressure and gas supply are normal, book a Gas Safe registered engineer. You can verify an engineer's registration at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- Coding resistor (R1) for Protherm control board · from £20
- Protherm PCB / control board (Gepard / Panther series) · 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 £120–£380, depending on the underlying cause.
Frequently asked questions
What is the difference between Protherm fault codes F.42 and F.43?
F.42 indicates a direct problem with the coding resistor — for example, the resistor is open-circuit or giving a completely absent reading. F.43 means the value being read is present but does not match any recognised boiler variant. In practice the two codes share the same root causes (coding resistor failure, incorrect installer settings, or a damaged PCB) and a Gas Safe engineer will diagnose both in the same way.
Can I fix Protherm F.43 myself?
The only owner-safe steps are resetting the boiler once or twice, checking system pressure, and confirming the gas supply is on. Everything beyond that — testing the coding resistor, accessing the installer menu, inspecting the PCB — must be done by a Gas Safe registered engineer. Attempting to open the boiler casing or tamper with the control board yourself is unsafe and will likely void any warranty.
How much does it cost to fix F.43 on a Protherm boiler in the UK?
If the coding resistor alone is at fault, repair costs are typically in the range of £120–£220 once parts and labour are combined. If the full PCB needs replacing, the bill rises considerably — most people pay £300–£380, though in some cases a PCB and labour can push beyond £450 depending on the engineer's rates and your location. London and the South East tend to run 15–25% higher than the national average. If your boiler is over ten years old, it is worth asking the engineer whether a full boiler replacement would offer better long-term value.
Why does F.43 sometimes appear alongside F.70 on my Protherm boiler?
F.70 is a parameter or configuration error. When F.43 and F.70 appear together or alternate on the display, it strongly suggests that the boiler variant code in the installer menu is incorrect or has been lost — for example after a power cut or following a PCB swap. An engineer correcting the product code setting in the installer menu will usually resolve both codes simultaneously.