Protherm F.00 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.00 fault code mean?
The F.00 code appears on Protherm boilers when the electronic control unit stops receiving a valid signal from the heating flow temperature sensor, known as NTC2. This sensor sits on the heating flow pipe and continuously feeds temperature data back to the boiler's PCB. When that signal disappears — whether because the sensor has failed, its wiring has come loose, or the circuit is broken — the boiler treats it as a safety-critical condition and shuts down ignition completely, leaving you with no heating or hot water. On the display, the letter 'F' will flash alongside the code '00'. Related codes F.01, F.10, and F.11 follow the same diagnostic logic but target different sensors (return and secondary circuits); if you see one of those instead, the approach described here still applies.
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 NTC2 flow sensor Common
The NTC2 thermistor itself has developed an open circuit or its resistance has drifted so far out of specification that the PCB cannot make sense of the reading. This is the most common resolution — the sensor is an inexpensive component and does wear out over time, particularly in hard-water areas where scale build-up causes localised overheating.
- Loose, corroded, or damaged wiring connector Common
The plug or terminal block connecting the NTC2 sensor to the wiring harness has worked loose, suffered moisture ingress, or has oxidised contacts. Vibration from the boiler over many years is often enough to cause an intermittent or permanent break in the connection, even if the sensor itself is healthy.
- Chafed or broken cable in the wiring loom Sometimes
The cable running from the NTC2 sensor back to the control board may have rubbed against a hot surface, been pinched during a previous repair, or simply cracked with age. A break anywhere in that cable produces the same open-circuit reading as a dead sensor.
- Mains power quality issues Sometimes
Voltage surges or significant dips on the household supply can cause the boiler's electronics to misinterpret sensor data or latch a fault even when the sensor is physically fine. A single reset after a power event will often clear this, but recurring grid instability can cause repeated lockouts.
- PCB fault affecting sensor input circuit Rare
If the sensor and all its wiring check out correctly, the input circuit on the printed circuit board itself may have failed. This is a less common but more expensive root cause, typically only confirmed after the sensor and wiring have been ruled out.
How to fix it
- Check for a recent power cut or voltage surge DIY safe
If your home had a power interruption recently, the fault may have been triggered by electrical noise rather than a genuine sensor failure. Try pressing the boiler's reset button once. If the boiler fires up and runs normally, monitor it over the next day or two. Do not reset more than two or three times in total — repeated resets without a fix can mask a worsening problem.
- Verify the boiler's system pressure is in the normal range DIY safe
Check the pressure gauge on the boiler front panel. Normal operating pressure for most Protherm models is between 1.0 and 1.5 bar when cold. A very low pressure reading (below 0.5 bar) can sometimes contribute to erratic sensor readings. If the pressure is low, top it up carefully via the filling loop until it sits around 1.0–1.2 bar, then reset the boiler.
- Inspect visible wiring and connectors at the sensor location DIY safe
You can safely look (without touching live components) at the area around the heating flow pipe where the NTC2 sensor is clipped on. Check whether any wiring appears obviously detached, melted, or heavily corroded. Do not disconnect or reconnect anything while the boiler is powered. Report what you observe to the engineer — it may speed up the diagnosis.
- Have a Gas Safe engineer test the NTC2 sensor resistance Gas Safe engineer
An engineer will isolate the boiler, disconnect the NTC2 sensor, and measure its resistance with a calibrated multimeter. The reading is compared against the manufacturer's temperature-resistance table. A significant deviation confirms the sensor needs replacing. This is not a DIY task — accessing the sensor involves working inside the boiler casing.
- Have the engineer inspect and test the full wiring loom Gas Safe engineer
If the sensor tests within specification, the engineer will trace the cable from the sensor back to the PCB, checking for continuity breaks, chafed insulation, and corrosion at every connector. Any damaged section of loom will be repaired or replaced.
- Arrange sensor replacement if confirmed faulty Gas Safe engineer
The NTC2 sensor is a low-cost part (typically under £30 from UK spares suppliers), and replacement is usually straightforward for an engineer familiar with Protherm boilers. After fitting, the engineer should run the boiler through a full heat cycle to confirm the fault has cleared and no related codes (F.01, F.10, F.11) are present.
- Request PCB investigation if the fault persists after sensor and wiring work Gas Safe engineer
If the new sensor and clean wiring still produce an F.00 lockout, the engineer will investigate the PCB's sensor input circuit. PCB repair or replacement is a more involved and costlier job — ensure the engineer confirms this diagnosis before committing to the part cost.
- Call a Gas Safe registered engineer if you cannot clear the fault yourself Gas Safe engineer
If the boiler will not reset, or if it relocks shortly after resetting, do not continue attempting resets. Contact a Gas Safe registered engineer to carry out a full diagnosis. You can verify an engineer's registration at gassaferegister.co.uk. All work on internal boiler components in the UK must be carried out by a registered professional.
Parts you may need
- NTC2 flow temperature sensor (thermistor) · from £20
- Sensor wiring loom / harness section · from £35
- Sensor connector plug and terminals · from £12
- Protherm PCB (control board) · 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 £90–£220, depending on the underlying cause.
Frequently asked questions
Can I fix Protherm F.00 myself by resetting the boiler?
A single reset is always worth trying, especially if there was a recent power cut or voltage blip on your supply. However, if the boiler relocks with F.00 again within a short time, resetting is not fixing the underlying problem. Do not reset more than two or three times. The actual repair — testing and replacing the NTC2 sensor or its wiring — requires a Gas Safe registered engineer to work inside the boiler casing.
How much does it cost to fix a Protherm F.00 fault in the UK?
For the most common cause — a failed NTC2 sensor — you are typically looking at £90 to £220 all in, covering the engineer's call-out, labour, and the part (which is usually under £30). If the wiring loom needs repair the cost sits at the higher end of that range. In the rarer situation where the PCB's sensor input circuit has failed, a replacement PCB can add £200–£350 to the bill, but this outcome is much less common and should only be confirmed after the sensor and wiring have been fully tested and cleared.
What is the difference between Protherm fault codes F.00, F.01, F.10, and F.11?
All four codes point to temperature sensor problems but on different parts of the heating circuit. F.00 relates to the heating flow sensor (NTC2) being disconnected or out of range. F.01 is the equivalent fault for the return circuit sensor. F.10 and F.11 indicate an open or short circuit respectively on the same sensors rather than a simple disconnection. The diagnostic approach — checking the sensor, its connectors, and the wiring back to the PCB — is the same for all four, so if you see any of these codes the steps on this page will guide you correctly.
Why does my Protherm keep showing F.00 after the sensor was already replaced?
If F.00 returns after a new sensor has been fitted, the most likely explanations are a fault in the wiring between the sensor and the PCB (a break or high-resistance joint that wasn't found during the first visit), a faulty replacement part, or — less commonly — a problem with the PCB input circuit itself. Ask your engineer to re-test the full wiring loom for continuity and to verify the resistance of the new sensor while it is connected to the harness, not just in isolation.