Protherm F.04 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.04 fault code mean?
The F.04 code appears on Protherm boilers when the control board detects a short circuit in the domestic hot water (DHW) storage tank temperature sensor — the NTC thermistor responsible for monitoring hot water temperature. Rather than an open circuit (a broken wire), F.04 specifically indicates the sensor signal has collapsed in a way the board reads as a short. When this happens, the boiler typically continues to provide central heating, but hot water production is suspended until the fault is resolved. On compatible Protherm models, the letter F will flash on the display alongside the error number. Note that a broken sensor circuit alone does not trigger F.04 — the board interprets a fully open circuit as simply 'no storage tank connected', so F.04 is reserved for the short-circuit condition specifically.
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 short-circuited DHW NTC temperature sensor Common
The NTC thermistor in the hot water storage tank is the primary culprit. Over time these sensors can develop internal shorts, particularly if moisture ingress or scale build-up has degraded the sensor body or its connections. A shorted sensor sends a misleading resistance reading to the control board, triggering F.04.
- Damaged or pinched sensor wiring Common
The wiring loom between the DHW sensor and the control board can become pinched, chafed, or corroded, causing two conductors to touch and create a short circuit. This is especially common where cables pass through panel edges or have been disturbed during previous servicing.
- Loose or corroded sensor connector Sometimes
The plug-in connector at either the sensor end or the PCB can develop corrosion or work loose, causing intermittent short-circuit signals. This is worth checking before condemning the sensor itself.
- Power supply surge or voltage instability Sometimes
A voltage spike or an unstable mains supply can temporarily corrupt sensor readings and trigger fault codes including F.04. If the code appeared after a power cut or electrical storm and clears on reset, this is the likely explanation.
- PCB (control board) fault Rare
If the sensor and its wiring test as healthy, the fault may lie with the PCB itself misreading the sensor input. PCB failure is less common but should be considered when all other checks draw a blank.
- Gas valve or ignition-related fault Rare
Some Protherm firmware versions can log F.04 alongside combustion issues such as a contaminated ionisation electrode or a gas valve problem. If the boiler is also struggling to fire or hold a flame, these secondary causes deserve investigation.
How to fix it
- Check whether hot water or heating has been affected DIY safe
Note what the boiler is still doing. F.04 typically keeps central heating running but disables hot water. Confirming this helps narrow down the fault to the DHW sensor circuit rather than a wider system problem.
- Reset the boiler once or twice DIY safe
Follow your boiler's reset procedure (usually holding the reset button for a few seconds). If the fault clears and stays clear, a temporary voltage fluctuation was likely the cause. If F.04 returns within a short time, a reset alone will not fix it — do not keep resetting repeatedly as this can mask developing faults.
- Check your mains gas supply DIY safe
Make sure other gas appliances in your home (hob, gas fire) are working normally. If there is no gas elsewhere, contact your gas supplier rather than a boiler engineer.
- Inspect the DHW sensor and its wiring — engineer required Gas Safe engineer
A Gas Safe engineer will locate the NTC sensor on the hot water storage tank, disconnect it safely, and test its resistance with a multimeter across the relevant temperature range. A shorted sensor will show near-zero resistance rather than the expected several kilohms. They will also trace the wiring loom back to the PCB connector, checking for chafed insulation, pinched cables, or corroded terminals.
- Replace the DHW NTC temperature sensor if faulty — engineer required Gas Safe engineer
If the sensor tests outside its correct resistance range or shows a short, the engineer will fit a compatible replacement NTC sensor. On Protherm systems this is a straightforward part swap, though the storage tank may need to be partially drained first depending on sensor location.
- Check and repair wiring or connectors if sensor tests healthy — engineer required Gas Safe engineer
If the sensor itself reads correctly in isolation, the engineer will check the full wiring run and connector integrity. A damaged section of cable or a corroded plug can be repaired or replaced without needing a new sensor.
- Assess the PCB if all other components are sound — engineer required Gas Safe engineer
Should the sensor and wiring prove fault-free, the engineer may test the PCB's sensor input circuit. A failed input channel on the board is less common but will require PCB repair or replacement by a qualified professional.
- Call a Gas Safe registered engineer if the fault persists after a reset Gas Safe engineer
If F.04 returns after one or two resets, book a Gas Safe registered engineer to carry out a proper diagnostic. Attempting to probe sensors, connectors, or the PCB without the correct training and tools risks electric shock, water damage, and — where gas components are involved — a safety hazard. Only a Gas Safe engineer may legally work on gas-carrying components.
Parts you may need
- DHW NTC temperature sensor (storage tank) · from £35
- Sensor wiring loom / harness · from £25
- Sensor connector plug and terminals · from £10
- Protherm PCB (control board) · 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 £100–£300, depending on the underlying cause.
Frequently asked questions
Can I still use my central heating while F.04 is showing?
In most cases, yes. Protherm boilers displaying F.04 generally continue to run the central heating circuit normally — it is the domestic hot water function that is suspended. That said, you should arrange a repair promptly rather than running the boiler indefinitely with a live fault, as underlying wiring or PCB issues can worsen over time.
How much does it cost to fix Protherm F.04 in the UK?
The most common repair — replacing the DHW NTC sensor — typically costs £80–£180 all in, covering the part and a standard engineer call-out. If the fault turns out to be a wiring issue, expect a similar or slightly lower bill. A PCB replacement is less common but can cost £300–£450 including labour, and is worth factoring into a decision about whether to repair or replace an older boiler.
Why has F.04 appeared after a power cut?
A mains voltage surge or sudden power restoration can temporarily upset the sensor signal and trigger F.04. Try resetting the boiler once or twice — if the code clears and does not return, a power fluctuation was most likely the cause. Installing a voltage stabiliser or UPS (uninterruptible power supply) is worth considering if power quality in your area is poor, as it protects the PCB and sensors from future spikes.
How do I prevent F.04 from recurring?
An annual boiler service by a Gas Safe registered engineer is the most effective preventive measure. During a service the engineer will inspect the sensor and its connections, clean the burner and electrodes, and check for any early signs of wiring deterioration — catching problems before they cause a breakdown. A service typically costs £80–£130 in the UK and is far cheaper than an emergency call-out.