Protherm F5 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F5 fault code mean?
The F5 code on most Protherm boilers — including the Panther, Leopard, and Gepard — signals a problem with the outdoor temperature sensor used for equithermal (weather compensation) control. This sensor feeds external temperature readings to the boiler so it can automatically adjust the flow temperature to match outside conditions. When the boiler detects that the sensor connection is broken, the sensor has failed, or the readings are implausible, it raises F5 and shuts down. Note that F5 only appears if an outdoor sensor is physically wired to your boiler — if you have no outdoor sensor installed, this code should not appear. On the Protherm Lynx Combi, F5 has a different meaning entirely: it indicates an airflow or flue fan fault. Always check which model you have before following any diagnostic steps.
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 outdoor NTC temperature sensor Common
The outdoor sensor itself has developed an internal fault — either a short circuit giving the control board an implausible temperature reading, or an open circuit giving no reading at all. This is the most common root cause of F5 on Panther, Gepard, and Leopard models.
- Broken or damaged sensor wiring Common
The cable running from the outdoor sensor into the boiler is exposed to the elements and can deteriorate over time. A single broken wire in the cable is enough to interrupt the signal and trigger F5, even if the sensor itself is perfectly healthy.
- Corroded or loose wiring connections Sometimes
Oxidation or a loose terminal at the point where the sensor cable connects to the boiler can produce an intermittent or absent signal. This is especially common on older installations where the connections have never been inspected.
- Airflow or flue fan fault (Lynx Combi only) Sometimes
On the Protherm Lynx Combi, F5 does not relate to an outdoor sensor at all — it indicates a problem with the air pressure or fan proving circuit. This could involve a blocked flue, a faulty pressure switch, or a failing fan and is an entirely different fault requiring different diagnostic steps.
- Control board failure Rare
In rare cases the boiler's PCB misinterprets the sensor signal or fails to process it correctly, generating a false F5. This is only suspected once the sensor, wiring, and connections have been checked and found to be in good order.
How to fix it
- Identify your exact Protherm model DIY safe
Check the data plate on the front or inside the casing of the boiler. Confirm whether you have a Panther, Gepard, Leopard, or Lynx Combi — and whether you actually have an outdoor sensor connected. If you have a Lynx Combi, the F5 fault relates to airflow rather than a sensor, and you should not attempt any of the outdoor sensor checks below.
- Attempt a single boiler reset DIY safe
Press and hold the Reset button on the boiler's control panel as described in your user manual. If F5 clears and the boiler fires up normally, monitor it over the next few hours. If the code returns, a reset is not the fix — move on to the next steps.
- Check whether the outdoor sensor can be temporarily disconnected to allow heating to continue DIY safe
On many Protherm models, disabling or disconnecting the equithermal function in the boiler's settings allows the boiler to revert to standard panel-controlled operation using its internal sensor. This is not a repair — it is a temporary workaround to keep your home warm while you arrange an engineer visit. Refer to your boiler's user manual for the relevant menu setting, or ask your engineer to configure this remotely.
- Visually inspect the outdoor sensor and its cable DIY safe
You can visually check the sensor unit on the outside of your property and the cable running back to the boiler for obvious damage — such as a cable that has been cut, pinched, or has a section of exposed or corroded wire. Do not attempt to open the boiler casing or interfere with the wiring terminals inside the boiler.
- Have a heating engineer test the sensor and wiring Gas Safe engineer
A qualified heating engineer will use a multimeter to check the resistance of the outdoor NTC sensor across the expected temperature range, and to verify continuity throughout the sensor cable. This confirms whether the sensor, the cable, or both need replacing.
- Have a heating engineer replace the faulty sensor and/or repair the wiring Gas Safe engineer
If the sensor or cable is confirmed faulty, the engineer will source and fit a replacement outdoor NTC sensor compatible with your Protherm model. They will also clean or re-make any corroded connection points and re-enable equithermal control before testing the boiler through a full heating cycle.
- If the fault persists after sensor replacement, have the PCB investigated Gas Safe engineer
Should F5 reappear after a new sensor and wiring have been fitted, the engineer will need to examine the control board for signs of damage or component failure. PCB diagnosis and replacement must only be carried out by a suitably qualified engineer.
- Contact a Gas Safe registered engineer if you are unable to resolve the fault yourself Gas Safe engineer
Because Protherm boilers are less common in the UK than brands such as Worcester Bosch or Ideal, it is worth contacting either the Protherm UK service network or an independent Gas Safe registered engineer experienced with continental European boiler brands. Parts availability can sometimes be slower, so having a professional source the correct sensor will save time.
Parts you may need
- Outdoor NTC temperature sensor (equithermal/weather compensation sensor) · from £45
- Sensor wiring harness / extension cable · from £20
- Boiler control PCB (Protherm Panther / Leopard) · from £180
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–£260, depending on the underlying cause.
Frequently asked questions
Will my Protherm boiler work without the outdoor sensor?
On many Protherm models, yes — as a temporary measure. The boiler can usually be configured to operate using only its internal flow temperature sensor, ignoring the outdoor unit entirely. This means you lose the energy-saving benefit of weather compensation, but the boiler can continue heating your home while you arrange a repair. Check your user manual or ask your engineer to disable the equithermal function in the settings menu.
Is F5 on a Protherm Lynx Combi the same fault?
No. On the Lynx Combi, F5 indicates an airflow or fan-proving fault rather than an outdoor sensor problem. This is a more complex fault that can involve a blocked or restricted flue, a faulty air pressure switch, or a failing fan. It requires a Gas Safe registered engineer to diagnose and must not be confused with the outdoor sensor fault seen on Panther, Gepard, and Leopard models.
How much does it cost to fix a Protherm F5 fault in the UK?
For the outdoor sensor fault on Panther, Leopard, or Gepard models, most homeowners pay somewhere between £100 and £260 all in — covering the engineer's callout, the replacement sensor, and labour. The outdoor NTC sensor itself is a relatively inexpensive part, typically £30–£70, so this sits at the lower end of boiler repairs. Bear in mind that Protherm parts can occasionally take longer to source in the UK than parts for more common brands, which may affect the overall cost and turnaround time. If the fault turns out to involve the PCB rather than the sensor, costs can rise considerably — your engineer will advise you once diagnosis is complete.
Can I prevent F5 from coming back?
The best prevention is an annual boiler service carried out by a Gas Safe registered engineer. During a service, the engineer will inspect the outdoor sensor, check the cable for any deterioration, and clean the connection terminals — catching the kind of slow corrosion or cable wear that typically causes F5 before it becomes a breakdown. If your sensor is more than several years old and F5 has appeared more than once, proactively replacing it at service time is often the most cost-effective approach.