Protherm F.91 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.91 fault code mean?
The F.91 fault code on a Protherm boiler signals a problem with the APC (Automatic Pump Control) module's NTC temperature sensor, which monitors an external domestic hot water (DHW) storage cylinder. In plain terms, the boiler has detected a short circuit in that sensor circuit — specifically, the voltage across the sensor has dropped below the 0.45V threshold that the control system uses to flag a fault. On Gepard and Panther series models, F.90 means the APC module itself is not communicating at all, while F.91 means the module is present but its sensor has failed. On Protherm models that share Vaillant's platform architecture, F.91 (often appearing alongside F.12) similarly points to a short circuit in the cylinder temperature sensor, its wiring harness, or casing damage. The flashing 'F' symbol on your display alongside '91' will remain until the root cause is resolved — although if the fault clears on its own within about 10 seconds, the issue may have been a temporary voltage drop rather than a permanent failure.
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
- Short-circuited NTC sensor on the external DHW cylinder Common
The most frequent trigger for F.91. The NTC sensor fitted to the external hot water cylinder develops an internal short circuit, causing the voltage in its circuit to fall below 0.45V. The boiler interprets this as a sensor failure and locks out the APC function. The sensor itself is a small, relatively inexpensive component but it does need correct electrical testing and replacement by an engineer.
- Damaged or loose wiring between the sensor and the boiler Common
Even if the NTC sensor is in good condition, chafed insulation, a pulled connector, or a pinched cable in the harness running between the cylinder sensor and the boiler's control board can create the same low-voltage short-circuit reading. Moisture or corrosion inside a connector plug is a particular culprit, especially if the external cylinder is in a damp utility room or airing cupboard.
- APC module failure Sometimes
The APC module itself — the add-on unit that manages the pump control for the external DHW system — can fail internally. Where F.90 points to the module being completely absent or disconnected, F.91 can indicate the module is present but has developed an internal fault that makes it misread or misreport the sensor signal. Module replacement is more involved and parts can be harder to source for Protherm than for more common UK brands.
- Faulty main PCB (control board) Rare
If an engineer has confirmed that the NTC sensor resistance is within specification and the wiring shows no damage, but F.91 persists even after fitting a new sensor, the main control board is likely misinterpreting the incoming signal. PCB failure is the least common cause of this specific code but should be investigated once cheaper components have been ruled out.
How to fix it
- Check whether the fault clears itself within 10 seconds DIY safe
Protherm's control logic will automatically cancel the F.91 alert if the sensor voltage recovers within roughly 10 seconds, suggesting the fault was a brief transient rather than a hard failure. Note the time the code appeared and watch the display. If the fault disappears on its own and does not return, monitor the boiler over the next few days.
- Reset the boiler (up to two or three times) DIY safe
Locate your boiler's reset button — usually a dedicated button or a mode-selection option on the display panel — and follow the instructions in your user handbook. A soft reset can clear a one-off sensor glitch. Do not reset more than two or three times in quick succession; repeated resets without the fault clearing indicate a genuine component problem that needs an engineer.
- Inspect the NTC sensor wiring for obvious damage DIY safe
You can safely look at the cable running from your external hot water cylinder back to the boiler. Check for any visible kinks, cuts, or sections of cable that look crushed or have exposed copper. Also look at the connector plug at both ends for signs of corrosion, moisture, or a connector that has partially pulled free. Do not unplug or open any components — just observe and report what you find to your engineer.
- Have a Gas Safe engineer test the NTC sensor resistance Gas Safe engineer
An engineer will disconnect the NTC sensor and measure its resistance with a multimeter. Each sensor has a known resistance-temperature relationship; a reading that falls outside the expected range confirms the sensor is faulty. This is a straightforward test but requires opening electrical components and working on boiler circuitry, so it must be carried out by a qualified professional.
- Have the engineer replace the NTC sensor if it is faulty Gas Safe engineer
If the resistance test confirms the sensor has failed, it should be replaced with a compatible Protherm part. After fitting the new sensor the engineer will power the boiler on to confirm F.91 has cleared and that the cylinder temperature is being read correctly. Note that Protherm parts can take longer to source in the UK than Worcester Bosch or Vaillant components, so allow extra time if your model is older.
- Have the engineer inspect and test the APC module Gas Safe engineer
If the sensor and wiring test fine, the engineer should turn attention to the APC module itself — checking its connections, testing its outputs, and comparing them with the expected specifications. A failed APC module will need to be replaced. The engineer should confirm the replacement module is correctly configured for your specific DHW cylinder setup before signing off.
- Have the PCB inspected or replaced if all other components check out Gas Safe engineer
As a last resort, if a new sensor and a confirmed-good APC module still produce F.91, the main control board is the most likely remaining culprit. PCB replacement is the most expensive repair on this list, so a reputable engineer will only recommend it after ruling out the simpler causes above. Always get the diagnosis and the cost of the PCB repair in writing before authorising the work.
- Call a Gas Safe registered engineer if the fault does not resolve after resetting Gas Safe engineer
If the boiler continues to display F.91 after one or two resets, book a Gas Safe registered engineer to carry out a full diagnosis. You can verify any engineer's registration at gassaferegister.co.uk. Given that Protherm is a less widely serviced brand in the UK, it is worth asking whether the engineer has experience with Protherm or Vaillant-platform boilers before booking.
Parts you may need
- Protherm NTC cylinder temperature sensor · from £30
- APC module wiring harness / connector · from £25
- Protherm APC module · from £280
- Protherm main PCB (control board) · from £320
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 £100–£320, depending on the underlying cause.
Frequently asked questions
Is the Protherm F.91 code dangerous — do I need to turn my boiler off immediately?
F.91 is not a gas-safety or carbon monoxide alert, so it does not require you to shut off the gas and evacuate. It relates to the sensor monitoring your external hot water cylinder. However, while the fault is active your boiler may not heat the DHW cylinder correctly, so you could be left without hot water. Reset the boiler once or twice as described above, and if the fault returns book an engineer promptly rather than leaving it unresolved for weeks.
What is the difference between Protherm F.90 and F.91?
On Gepard and Panther series boilers, these two codes relate to the same APC (Automatic Pump Control) module but describe different problems. F.90 means the boiler cannot detect the APC module at all — typically a connection or communication issue between the module and the boiler. F.91 means the module is connected and recognised, but its NTC temperature sensor is reporting a short circuit. In practice, F.90 is often a loose plug, while F.91 usually means the sensor or its wiring has failed.
How much does it typically cost to fix Protherm F.91 in the UK?
If the fault turns out to be a faulty NTC sensor — the most common cause — you can expect to pay roughly £100–£200 all-in for parts and an hour or so of labour. If the APC module itself needs replacing, costs typically climb to £250–£400 depending on parts availability and your location. A PCB replacement is at the higher end and can cost £350–£500 or more; if your boiler is approaching 10–15 years old and needs a new PCB, it is worth getting a replacement boiler quote for comparison. Prices in London and the South East tend to run 15–25% higher than the national average.
Can I use my boiler while F.91 is showing?
The boiler may still provide central heating even when F.91 is active, but the APC-controlled domestic hot water function is likely to be impaired or disabled. Continuing to run the boiler while repeatedly resetting the fault without having it investigated is not recommended — it masks the underlying problem and could mean your cylinder is not being heated safely or efficiently. Have an engineer look at it sooner rather than later.