Protherm F.80 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.80 fault code mean?
The F.80 code appears on Protherm boilers when the control board detects a fault with the temperature sensor monitoring the inlet side of the Secondary Heat Exchanger (SHE) — the plate heat exchanger responsible for producing domestic hot water. In plain terms, the boiler cannot get a reliable temperature reading from that sensor, so it shuts down to protect itself. On Protherm displays, the letter F flashes alongside the error number. You can review the ten most recent fault codes on the boiler by pressing both CH temperature adjustment buttons simultaneously for around seven seconds — useful for spotting whether F.80 is a one-off or a recurring issue.
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 or degraded NTC temperature sensor Common
The inlet NTC sensor on the secondary heat exchanger is the most common culprit. Over time these sensors drift out of tolerance or fail entirely, giving the control board a reading it cannot trust. When the measured resistance no longer matches the expected value for the water temperature, the board raises F.80 and locks the boiler out.
- Damaged wiring or loose connector Common
The wiring harness between the SHE inlet sensor and the PCB is exposed to heat cycling and vibration over years of use. Corroded pins, a cracked wire, or a connector that has worked loose can interrupt the signal in exactly the same way as a failed sensor, triggering the same fault code.
- Short circuit or open circuit in the sensor loop Sometimes
A short to earth or a broken wire causing an open circuit in the sensor loop produces an out-of-range resistance reading. Engineers typically use a multimeter to check continuity and resistance before condemning the sensor itself — an open circuit with an otherwise undamaged sensor points straight to the wiring.
- Faulty PCB (control board) Rare
If the sensor and all associated wiring test within specification, the problem may lie with the PCB itself. A failed input channel or dry solder joint on the board can misinterpret a perfectly good sensor signal. PCB faults are less common but should be considered once all other components have been ruled out.
How to fix it
- Check for a flashing F on the display and note the full code DIY safe
Confirm the display is showing F.80. The flashing F confirms this is an active fault rather than a historical log entry. If you want to check whether the code has appeared before, press both CH temperature buttons together for seven seconds to scroll through the last ten faults.
- Attempt a single boiler reset DIY safe
Locate the reset button (often a flame symbol or dedicated reset key on the control panel) and hold it for the duration described in your boiler's quick-start guide — typically around three seconds. If the boiler fires and runs normally, monitor it closely over the next hour. If F.80 returns promptly, do not keep resetting; repeated resets on a persistent lockout can mask a genuine fault.
- Do not attempt to inspect or replace the sensor yourself Gas Safe engineer
The SHE inlet NTC sensor and its wiring are internal components. Gaining access involves removing boiler casing, and the repair sits alongside the gas circuit. This work must only be carried out by a Gas Safe registered engineer.
- Arrange for a Gas Safe engineer to inspect the sensor and wiring Gas Safe engineer
The engineer will remove the relevant casing panels, locate the SHE inlet sensor, and visually inspect the connector and wiring for corrosion, cracking, or looseness. Many F.80 faults are resolved at this stage by reseating a connector or replacing a short section of damaged wire.
- Engineer to test NTC sensor resistance Gas Safe engineer
Using a multimeter, the engineer measures the sensor's resistance at a known temperature and compares it against Protherm's specification for the Gepard range. A reading outside tolerance confirms the sensor has failed and needs replacing.
- Engineer to replace the SHE inlet NTC sensor if faulty Gas Safe engineer
Replacement NTC sensors for the Gepard series are relatively inexpensive parts. Once fitted, the engineer will clear the fault, run the boiler through a full domestic hot water demand cycle, and confirm the F.80 code does not return.
- Engineer to test or replace the PCB if sensor and wiring are sound Gas Safe engineer
If the sensor resistance and all wiring check out correctly but F.80 persists, the PCB is the likely cause. The engineer will assess whether the board can be repaired or needs full replacement, and advise on cost versus the boiler's age before proceeding.
- Call a Gas Safe registered engineer if the fault persists after a reset DIY safe
F.80 is not a fault a homeowner can resolve beyond a single reset attempt. Book a Gas Safe registered engineer to diagnose and repair the fault safely. You can verify any engineer's registration at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- SHE inlet NTC temperature sensor (Gepard series) · from £22
- Wiring harness / sensor connector loom · from £35
- PCB (control board) for Protherm Gepard · from £260
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.80 myself?
No. While you can safely try a single reset to see whether F.80 clears on its own, the underlying fault — a failed sensor, damaged wiring, or faulty PCB inside the boiler — requires a Gas Safe registered engineer to diagnose and repair. Opening the boiler casing and working near the gas circuit without Gas Safe registration is illegal and dangerous in the UK.
How much does it cost to fix a Protherm F.80 fault?
Most F.80 repairs fall between £150 and £400 including parts and labour. A straightforward NTC sensor replacement typically costs £100–£200 all in, while a wiring repair sits in a similar range. If the PCB turns out to be the cause, parts alone can be £200–£300, pushing the total towards £400–£500. It is worth asking your engineer for a written quote before authorising any PCB replacement, especially on older Gepard boilers where the repair cost may approach the value of the boiler.
Will F.80 affect my central heating as well as hot water?
Because the F.80 fault relates specifically to the secondary heat exchanger inlet sensor — the part of the boiler used for domestic hot water production — you might expect only your hot water to be affected. However, most Protherm boilers will lock out completely when this code appears, meaning both heating and hot water stop working until the fault is resolved and the boiler is reset by an engineer.
How do I check whether F.80 has happened more than once on my Protherm boiler?
Protherm Gepard boilers store the ten most recent fault codes in memory. To view them, press and hold both CH temperature adjustment buttons at the same time for approximately seven seconds. The display will cycle through the stored codes. If F.80 appears multiple times, it suggests a recurring or intermittent fault — useful information to share with your engineer before they begin diagnosis.