Protherm F.55 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.55 fault code mean?
The F.55 code appears on Protherm boilers when the built-in carbon monoxide (CO) sensor has detected a fault — either in the sensor itself, its associated wiring, or the control electronics that process its signal. Because CO is a colourless, odourless gas that can be lethal even in small concentrations, Protherm's firmware treats any anomaly here as a hard lockout: the boiler shuts down and will not restart until the fault is cleared. This is not a code to reset and ignore. If there is any possibility of real CO being present in your home, evacuate immediately and call the Gas Emergency line on 0800 111 999. F.55 sits within Protherm's 'F' error block, which covers serious safety-critical malfunctions, and it applies across the full range of current Protherm heating equipment.
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 CO sensor Common
The CO sensor itself is the most frequent culprit. Like all electrochemical sensors it has a finite service life and can degrade or fail outright, causing the boiler's PCB to register an out-of-range signal and trigger F.55. A straightforward sensor swap by a Gas Safe engineer will often clear the fault completely.
- Wiring or connector fault Common
Loose terminals, corroded connector pins, or a chafed wire between the CO sensor and the PCB can produce the same erratic signal as a dead sensor. The engineer will check continuity and inspect the wiring harness and plug connections before condemning any part.
- Faulty PCB or control board Sometimes
If the sensor and its wiring test correctly, the problem may lie with the electronic control board misreading or failing to process the sensor's output. A PCB fault can generate a false F.55 signal and will require board repair or replacement.
- Sticking or damaged relays on the PCB Sometimes
Relay contacts on the control board can stick or become pitted over time, disrupting the sensor circuit. An engineer may be able to clean and degrease affected contacts, although in many cases board replacement is the more reliable long-term solution.
- Genuine CO production triggering sensor response Rare
Occasionally, the sensor is working correctly and is responding to actual CO in the flue gases — caused by a blocked flue, a dirty or poorly adjusted burner, or inadequate ventilation to the boiler. This is the most dangerous scenario and must be treated as an emergency.
How to fix it
- Switch off the boiler and ventilate the property DIY safe
Turn the boiler off at its power switch. Open windows and doors in the boiler room and adjoining spaces. If anyone in the property feels unwell — headache, dizziness, nausea — leave the building immediately and call 999.
- Check your CO alarm DIY safe
If a CO alarm is sounding or shows a CO reading, evacuate everyone from the property, leave doors open as you go, and call the National Gas Emergency Service on 0800 111 999. Do not re-enter until the property has been declared safe by a qualified engineer.
- Do not attempt to reset the boiler repeatedly DIY safe
A single reset attempt is acceptable only if you are confident there is no CO present and no alarm has activated. If the F.55 code returns immediately, stop resetting — repeated resets will not fix an underlying sensor or wiring fault and could mask a genuine safety issue.
- Call a Gas Safe registered engineer for full diagnosis Gas Safe engineer
This fault requires hands-on investigation of the CO sensor, its wiring harness, connectors, and PCB — all internal gas appliance work that must legally be carried out by a Gas Safe registered engineer. You can verify any engineer's registration at GasSafeRegister.co.uk. Do not allow anyone unregistered to work on the boiler.
- Sensor replacement if confirmed faulty Gas Safe engineer
The engineer will test the CO sensor electrically. If it is out of specification or has failed completely, they will fit a manufacturer-compatible replacement sensor. This is the most common resolution for F.55.
- PCB or relay inspection and replacement if required Gas Safe engineer
If the sensor passes testing, the engineer will investigate the control board — checking relay contacts and board circuitry. Damaged relays may be cleaned; a faulty PCB will typically need to be replaced with a compatible unit to fully restore safe operation.
Parts you may need
- CO sensor (Protherm compatible) · from £45
- PCB / control board · from £220
- Wiring harness / connector kit · from £25
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–£350, depending on the underlying cause.
Frequently asked questions
Is the Protherm F.55 fault dangerous — should I leave the house?
Treat it seriously until proven otherwise. F.55 indicates a CO sensor fault, which could mean the sensor has simply failed electronically, or — in rarer cases — that it detected real carbon monoxide in the flue gases. If your CO alarm is sounding, or anyone feels unwell, evacuate immediately and call the Gas Emergency line on 0800 111 999. If there are no symptoms and no alarm, switch the boiler off, ventilate the room, and arrange for a Gas Safe engineer to attend as soon as possible. Do not keep running the boiler.
Can I reset the boiler to clear F.55?
One reset is reasonable if you are sure there is no CO alarm activation and no one feels unwell. However, F.55 is a hard safety lockout and will almost certainly return if the underlying fault — a failed sensor, a wiring issue, or a PCB problem — has not been fixed. Repeated resetting does not repair anything and could mask a genuine hazard. If the code comes back after one reset, call a Gas Safe engineer.
How much does it cost to fix a Protherm F.55 fault?
Most people with this fault pay in the region of £150–£350 all-in, covering the engineer's callout, diagnostic time, and parts. A straightforward CO sensor replacement sits at the lower end of that range. If the PCB also needs replacing, costs will be towards the top of the range or potentially above it — a Protherm-compatible control board alone can cost £200 or more in parts. Out-of-hours or emergency callouts will add to the labour cost, typically £120–£180 just for attendance.
Is it a legal requirement to have a CO alarm with a gas boiler?
Yes. Under UK regulations, a carbon monoxide alarm must be fitted in any room containing a fixed combustion appliance (gas boilers included, but not gas cookers). Beyond the legal requirement, a working CO alarm is your main line of defence against CO poisoning — CO is invisible and odourless, making it impossible to detect without one. Test your alarm monthly and replace it in line with the manufacturer's guidance, typically every 7–10 years.