Protherm F.30 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.30 fault code mean?
The F.30 code appears on Protherm boilers — displayed as a flashing 'F' alongside the number 30 — when the boiler's control system detects a failure with the lock-out sensor. This sensor acts as a high-limit safety device, monitoring the boiler for unsafe operating conditions such as excessive temperatures. When the boiler cannot read a valid signal from this sensor, it shuts itself down entirely as a precaution. The boiler will not restart on its own; the underlying fault must be identified and resolved before a manual reset will restore normal operation. F.30 falls within the F.20–F.42 block of Protherm fault codes, all of which relate to serious faults that affect both performance and operational safety. Note that the closely related code F.31 indicates a short circuit specifically within the lock-out sensor circuit, so if your display shows F.31 rather than F.30, a wiring or short-circuit fault near the sensor is the most likely starting point.
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
- Faulty or failed lock-out sensor Common
The lock-out sensor itself has deteriorated, corroded internally, or failed electrically. Protherm boilers are designed to treat any sensor reading that falls outside a realistic range as a fault, so even a partially degraded sensor can trigger F.30 before it fails completely. Sensor replacement is the most common outcome of an F.30 diagnosis.
- Damaged or loose wiring between the sensor and PCB Common
Corrosion, vibration, or heat exposure over time can loosen connectors or degrade the wiring that carries the sensor signal back to the control board. A broken or intermittent connection looks identical to a failed sensor from the boiler's perspective, making wiring inspection an early priority for any engineer attending this fault.
- Genuine overheating event triggering the sensor Sometimes
If the boiler genuinely overheated — due to a blocked heat exchanger, an airlock restricting water circulation, or a failing pump — the lock-out sensor may have responded correctly. In this scenario F.30 is the symptom of a deeper problem, and resolving just the sensor fault without investigating the root cause would likely result in the lockout recurring.
- Faulty PCB misreading the sensor signal Rare
The printed circuit board processes every signal the boiler receives, including those from the lock-out sensor. If the PCB develops a fault, it can misinterpret a normal sensor reading as a failure and trigger F.30 even when the sensor itself is in good working order. PCB issues are typically ruled out last, after the sensor and wiring have been tested.
How to fix it
- Reset the boiler once or twice DIY safe
Locate the reset button on your Protherm boiler (usually marked with a flame or reset symbol) and hold it for a few seconds. If F.30 clears and the boiler runs normally, monitor it closely over the next hour or two. If the code returns — or if the boiler fails to restart at all — do not keep resetting it, as repeated resets without fixing the root cause can mask a genuine safety issue.
- Check that your gas supply is active DIY safe
Confirm that other gas appliances in your home (such as a gas hob or fire) are working normally. If they are not, contact your gas supplier rather than attempting any boiler work. If gas supply is confirmed elsewhere, the issue is internal to the boiler.
- Check system pressure DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it has dropped below 1 bar, top it up using the filling loop — usually a braided flexible hose beneath the boiler with a small valve — until the gauge reads around 1.2 bar, then close the valve. Low pressure alone is unlikely to cause F.30, but it is worth ruling out before calling an engineer.
- Arrange for a Gas Safe engineer to inspect the lock-out sensor and its wiring Gas Safe engineer
A registered engineer will use a multimeter to test the sensor's resistance and continuity, comparing the readings against the manufacturer's specification. They will also inspect the wiring harness and connectors between the sensor and the PCB for signs of corrosion, heat damage, or loose terminals. In most cases, either the sensor or a wiring fault is identified and rectified at this stage.
- Have the engineer investigate for underlying overheating causes Gas Safe engineer
If the sensor appears to have operated correctly — i.e., it triggered in response to genuine high temperatures — the engineer will look further into why the boiler overheated. This includes checking the circulation pump, looking for airlocks in the system, and inspecting the heat exchanger for limescale or blockage. Addressing only the sensor without fixing the root cause will likely result in repeated lockouts.
- PCB inspection and replacement if required Gas Safe engineer
If the sensor and wiring are both found to be in good condition, the engineer will assess whether the PCB is misreading the sensor signal. PCB diagnosis is complex and is typically the last thing ruled out. If the PCB is confirmed faulty, the engineer will advise on replacement cost relative to the age and overall condition of the boiler — on an older appliance, a new boiler may be the more economical long-term choice.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
F.30 is not a fault that can be safely resolved through DIY work on internal components. All inspection, diagnosis, and replacement of the lock-out sensor, wiring, pump, heat exchanger, or PCB must be carried out by a Gas Safe registered engineer. You can verify any engineer's registration at the Gas Safe Register website (gassaferegister.co.uk) before work begins.
Parts you may need
- Lock-out / high-limit thermostat sensor · from £35
- Wiring harness / connector set · from £25
- Circulation pump · from £85
- Printed circuit board (PCB) · from £220
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
Can I fix Protherm F.30 myself?
The only safe DIY steps are a single boiler reset, confirming your gas supply is on, and checking that system pressure is within the normal 1–1.5 bar range. Any work beyond that — testing the sensor, inspecting wiring, checking the pump or heat exchanger, or replacing the PCB — must be carried out by a Gas Safe registered engineer. Working on internal gas appliance components without registration is illegal and potentially dangerous.
What is the difference between Protherm F.30 and F.31?
Both codes relate to the lock-out sensor circuit, but they indicate different fault types. F.30 means the control board has detected a general failure of the lock-out sensor — the signal is absent, invalid, or out of range. F.31 specifically points to a short circuit within the same sensor circuit, meaning current is flowing along an unintended path, usually due to damaged insulation or a faulty connector. An engineer attending either code will inspect the same components, but F.31 makes a wiring or connector short the most likely starting point.
Why does my Protherm keep showing F.30 after a reset?
If F.30 returns shortly after each reset, it means the underlying fault is still present and the boiler is simply detecting the same problem again. Common reasons include a sensor that has genuinely failed, a persistent wiring fault, or an ongoing overheating issue caused by a blocked heat exchanger or a weak circulation pump. Repeatedly resetting without an engineer's diagnosis can delay finding the real cause and risks masking a genuine safety concern.
How much does it cost to fix Protherm F.30 in the UK?
Most homeowners pay between £150 and £350 for an F.30 repair, which typically covers a Gas Safe engineer's callout, diagnosis, and replacement of the lock-out sensor or a wiring repair. If the cause turns out to be a more serious underlying issue — such as a faulty circulation pump — costs can extend towards the top of that range. PCB replacement is the exception: parts alone can reach £300–£500, so if your boiler is over 10 years old and needs a new PCB, it is worth getting a new boiler quote at the same time for comparison.