Protherm F.38 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.38 fault code mean?
The F.38 code appears on Protherm boilers when the control board (PCB) detects that a measured frequency signal falls outside its expected operating range. In practice, the PCB continuously monitors the fan's rotational speed via a frequency signal; if that signal strays too far from the acceptable band — whether too low, too high, or erratic — the boiler locks out as a safety precaution and displays F.38. Because the boiler will not fire without confirmation that the fan is running correctly, this fault causes a hard lockout until the underlying issue is resolved. F.38 sits in the F.20–F.42 range of Protherm fault codes, which the manufacturer classifies as serious malfunctions affecting both performance and operational safety. A single reset may clear a transient glitch, but if the code returns the boiler needs professional diagnosis.
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 worn fan motor Common
The fan (ventilator) is the most common trigger for F.38. As the motor's bearings wear or its windings degrade over time, the fan may spin too slowly, erratically, or not at all. Any of these conditions produces a frequency signal the PCB considers out of tolerance, prompting a lockout. A seized or noisy fan that struggles to reach operating speed is a clear sign this is the culprit.
- Loose or damaged fan wiring and connectors Common
The PCB relies on a clean electrical signal from the fan to confirm it is running at the correct speed. If the wiring between the fan and the PCB has worked loose, corroded, or been damaged — often by a slow water leak inside the boiler casing — that signal becomes intermittent or absent. The boiler then behaves as though the fan frequency is outside tolerance, even if the fan itself is mechanically sound.
- Faulty PCB Sometimes
If the fan and its wiring check out as healthy, the control board itself may be misreading or failing to process the frequency signal correctly. PCB faults can also cause the boiler to not send the correct voltage to the fan in the first place, making it difficult to tell at a glance whether the fan or the board is at fault. An engineer will use a multimeter to distinguish between the two.
- Mains electrical supply instability Rare
In rare cases, significant instability or noise on the incoming mains supply can affect the boiler's electronics and cause a spurious frequency reading. This is uncommon in normal UK domestic supplies but worth noting if the fault appears during periods of unusual electrical activity or alongside other appliance issues in the property.
How to fix it
- Reset the boiler once or twice DIY safe
Press the reset button as described in your boiler's user manual. A transient electrical glitch can occasionally trigger F.38, and a single reset may clear it. If the code returns within a short time, do not keep resetting — repeated resets will not fix a genuine component fault and may mask the problem for the engineer.
- Check that the gas supply is on DIY safe
Confirm your gas meter is on and that other gas appliances in the property are working normally. While a gas supply issue is not the direct cause of F.38, it is a quick sanity check before calling an engineer.
- Visually inspect around the boiler for signs of a water leak DIY safe
Look for damp patches, drip marks, or corrosion around the boiler casing. A slow internal leak is a known cause of wiring and connector damage. If you see evidence of a leak, note this and mention it to the engineer — it is useful diagnostic information.
- Have a Gas Safe engineer inspect and test the fan Gas Safe engineer
An engineer will remove the boiler casing, check whether the fan spins freely without resistance or grinding noise, and measure the frequency signal it produces. If the bearings are worn or the motor windings have failed, the fan will need to be replaced. In some cases, only the fan motor itself requires changing, which is cheaper than a full fan assembly replacement.
- Have the engineer inspect all wiring and connectors between the fan and PCB Gas Safe engineer
The engineer will check every connection in the fan circuit for looseness, corrosion, or heat or water damage. Loose connectors can often be re-seated and secured; damaged wiring sections will need to be replaced. While the casing is open it is sensible to ask them to check other connections at the same time.
- Have the engineer test the PCB Gas Safe engineer
Using a multimeter, the engineer will check that the PCB is supplying the correct voltage to the fan and processing the returning frequency signal accurately. If the board is at fault, they will advise on repair versus full replacement depending on the nature of the failure and parts availability.
- Replace the faulty component and carry out a full recommission Gas Safe engineer
Once the defective part — fan, wiring loom, or PCB — has been replaced, the engineer should run the boiler through a full heat and hot water cycle to confirm F.38 does not return and that all safety parameters are within range before leaving the property.
Parts you may need
- Boiler fan / ventilator assembly · from £135
- Fan motor (motor only, where separately available) · from £65
- Fan wiring harness / loom · from £40
- PCB (printed circuit board) · 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 £150–£400, depending on the underlying cause.
Frequently asked questions
Can I fix Protherm F.38 myself?
No. Unlike a frozen condensate pipe or low system pressure, F.38 relates to internal electrical and mechanical components — specifically the fan and PCB — that are only accessible by removing the boiler casing. Working inside a gas boiler without Gas Safe registration is illegal under UK gas safety regulations and potentially dangerous. Resetting the boiler once or twice is the only homeowner action that is appropriate.
How much does it typically cost to fix F.38 on a Protherm boiler?
Most people with this fault end up paying somewhere between £150 and £400 all in. A straightforward wiring repair sits at the lower end, while a fan replacement (parts and labour combined) usually comes in around £250–£350. If the PCB turns out to be the cause, a repair to the board can cost £250–£400, and a full PCB replacement can reach £450–£700 — though full PCB replacement is the less common outcome. Expect a diagnostic call-out charge of around £60–£100, which most engineers put towards the cost of the repair if you proceed with them.
Why does F.38 keep coming back after I reset the boiler?
A reset clears the lockout temporarily but does nothing to fix the underlying hardware. If the fan's bearings are worn, the motor is degrading, or a connector is loose, the frequency signal will fall out of tolerance again as soon as the boiler tries to fire. Repeated resets on this fault will not resolve it and may delay a proper diagnosis. If F.38 returns more than once or twice, stop resetting and book an engineer.
Is F.38 dangerous — should I stop using the boiler?
The boiler will have locked itself out, so it should not be operating. Do not attempt to bypass the lockout or force the boiler to run. The fault is a safety shutdown triggered because the control board cannot confirm the fan is working correctly — the fan is what removes combustion gases from the appliance. Until the fault is properly diagnosed and repaired by a Gas Safe engineer, the boiler should remain off.