Protherm F.15 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.15 fault code mean?
The F.15 fault code on a Protherm boiler points to a problem with the boiler's fan or its flue exhaust gas sensing system. When the electronic control unit detects that the fan's speed signal is missing or implausible, or that the exhaust gas sensor is returning an out-of-range or short-circuit reading, it flags F.15 and shuts the appliance down safely. On the display, the letter F flashes alongside the number 15. Depending on your specific Protherm model, the fault leans toward one of two interpretations: (1) a fan fault — where the fan motor is not spinning correctly, the tachometric (speed-feedback) signal is interrupted, or the 230 V supply to the fan is absent; or (2) an exhaust gas sensor short circuit — where the NTC sensor monitoring flue temperatures has developed a short circuit in its wiring or element. Both scenarios prevent safe combustion from being confirmed, so the boiler locks out until the root cause is identified and resolved. Because this code involves either the fan, the flue or gas-related sensing components, all diagnostic and repair work must be carried out by a Gas Safe registered engineer.
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
- Fan motor failure or seized impeller Common
The fan motor itself may have failed, or the impeller has become clogged with debris and dust, preventing it from reaching the correct speed. Insufficient lubrication of the drive shaft over time can also cause the motor to drag and drop below the minimum speed threshold the control board expects, triggering the fault.
- Tachometric signal wiring fault Common
The fan sends a continuous speed-feedback (tachometric) signal back to the PCB via a dedicated wiring connector. A broken wire, corroded terminal, or loose connector at the fan's multi-pin plug can interrupt this signal entirely, leading the boiler to assume the fan has stopped even when it is spinning normally.
- Exhaust gas sensor short circuit Sometimes
The NTC sensor positioned in the flue or exhaust path monitors outgoing gas temperatures. A short circuit within the sensor element or along its connecting wiring causes the control board to receive an implausible voltage reading. This is particularly common on Protherm models that share architecture with the wider Vaillant Group platform.
- Blocked or partially obstructed flue Sometimes
A build-up of debris, bird nesting material, or ice around the external flue terminal can restrict airflow. This may cause back-pressure that prevents the fan from spinning freely, or causes exhaust temperature readings to behave erratically, both of which can register as an F.15 condition.
- Mains voltage instability Sometimes
Significant fluctuations or dips in the household electricity supply can cause the fan's 230 V feed to drop below the level needed for normal operation. The control board interprets the resulting abnormal fan behaviour as a fault, even though the fan itself may be undamaged.
- PCB (control board) fault Rare
If the PCB's fan-control output stage or its sensor-monitoring circuitry becomes faulty, it may misread or fail to power the fan or sensor correctly. This is usually only confirmed after the fan and sensor have already been tested and found to be in good condition.
How to fix it
- Reset the boiler once or twice DIY safe
Locate the reset button on your Protherm boiler (typically marked with a flame or reset symbol) and hold it for the duration specified in your user guide. Allow the boiler a full minute to attempt a restart. A transient electrical glitch can sometimes latch the fault code, and a clean reset will clear it. Do not reset the boiler more than two or three times in total — repeated resets without the fault clearing can cause further wear and may mask a developing safety issue.
- Check that your gas supply is live DIY safe
Confirm that other gas appliances in your home (cooker, gas fire) are working normally. If they are not, there may be a wider gas supply issue; contact your gas network operator. If the gas supply is fine, this is unlikely to be the cause, but it is a worthwhile two-minute check before calling an engineer.
- Inspect the external flue terminal for blockages DIY safe
From outside the property, look at the flue outlet (the plastic or metal pipe exiting the wall or roof). Check for obvious obstructions such as bird nests, leaves, or ice — particularly in cold weather when condensate and moisture can freeze around the terminal opening. If ice is visible, you can carefully pour warm (not boiling) water over the terminal to thaw it. Do not reach inside the flue or attempt to dismantle any part of it.
- Check system pressure and top up if needed DIY safe
While not directly linked to F.15, a boiler that is low on water pressure (below 1 bar on the gauge) may behave unpredictably. If the needle sits below 1 bar, top up via the filling loop until it reads between 1 and 1.5 bar, then re-attempt a reset. This rules out a compounding pressure issue before the engineer arrives.
- Engineer to test the fan, its wiring and 230 V supply Gas Safe engineer
A Gas Safe engineer will check for 230 V at the fan's supply connector and verify that the tachometric signal wire is intact and correctly seated at the multi-pin connector. They will run the fan, measure the feedback signal, and compare the actual fan speed against the expected range stored in the control board. A faulty motor or seized bearing will be confirmed by direct measurement rather than assumption.
- Engineer to inspect and test the exhaust gas NTC sensor Gas Safe engineer
Using a multimeter, the engineer will measure the resistance across the exhaust gas sensor at a known temperature and compare it with the manufacturer's reference values. Wiring continuity from the sensor back to the PCB will also be checked. A short circuit typically shows as near-zero resistance across the sensor terminals and indicates the sensor element or its wiring needs to be replaced.
- Engineer to replace the faulty component Gas Safe engineer
Once the defective part is confirmed — whether that is the fan assembly, the exhaust gas NTC sensor, or associated cabling — the engineer will fit a genuine Protherm-compatible replacement. After fitting, they will run a full operational test to confirm the F.15 code has cleared and that combustion, flue flow, and exhaust temperatures are all within normal parameters.
- Engineer to assess the PCB if the fault persists Gas Safe engineer
If both the fan and the exhaust sensor test as serviceable and the wiring is undamaged, the fault may lie in the PCB's fan-drive circuitry or its sensor-input stage. PCB diagnosis requires specialist equipment and, in many cases, a replacement board will be needed. The engineer will confirm this diagnosis before ordering parts, as PCB replacement is one of the more expensive outcomes for this fault code.
Parts you may need
- Protherm fan assembly (model-specific) · from £95
- Exhaust gas NTC temperature sensor · from £20
- Fan wiring loom / connector harness · from £18
- PCB / control board (model-specific) · 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 £180–£380, depending on the underlying cause.
Frequently asked questions
Can I fix the Protherm F.15 fault myself?
Very little of this repair is suitable for a homeowner to tackle. You can safely try resetting the boiler once or twice, checking that your gas supply is on, and looking at the external flue terminal for obvious ice or blockages. Beyond that, the fan, its wiring, and the exhaust sensor all sit inside the appliance and involve either mains-voltage electrical components or gas-related sensing. Working on these yourself is unsafe and, where gas work is involved, illegal without Gas Safe registration. Call a Gas Safe registered engineer for anything beyond those basic checks.
How much does it cost to repair a Protherm F.15 fault in the UK?
For the most common outcomes — a fan replacement or an exhaust sensor swap including labour — most homeowners pay somewhere in the region of £180 to £380. A sensor replacement alone is towards the lower end of that range, while a fan motor replacement including parts and an hour or two of engineer time tends to sit in the middle. If the PCB turns out to be the culprit, costs can rise to £400–£550 or more depending on the board and your location, but this is a less common outcome and is usually only confirmed after simpler causes have been ruled out. Emergency or out-of-hours call-outs will carry an additional premium.
Why does the F.15 code come back after I reset the boiler?
If the fault returns within a short time of resetting, it means the underlying problem has not gone away. A reset simply clears the lockout and allows the boiler to retry — if the fan speed signal is still missing, the NTC sensor is still short-circuiting, or the flue is still blocked, the boiler will detect the same abnormal condition and lock out again. Repeated resets without a fix can mask a deteriorating component, so if F.15 returns after one or two resets, stop resetting and arrange for an engineer to diagnose the root cause.
Does F.15 mean my boiler's flue is dangerous?
F.15 is the boiler's way of telling you that something in the fan or exhaust monitoring system is not behaving as expected. The boiler shuts itself down as a precaution, which is exactly what it should do. This means combustion products are not being vented unsafely — the appliance has stopped rather than continuing to run with a compromised flue system. However, you should not attempt to override the lockout or keep force-resetting the boiler. Leave it locked out and get a Gas Safe engineer to inspect it promptly, especially if you have any smell of gas or unusual odours, in which case call the National Gas Emergency line on 0800 111 999 immediately.