Protherm F.64 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.64 fault code mean?
The F.64 code appears on Protherm boilers when the control board detects an ADC (Analogue-to-Digital Converter) error — in plain terms, the boiler's electronics cannot make reliable sense of the signals coming from the flow and/or return NTC temperature sensors. This typically happens because one of those sensors is short-circuited, because wiring between a sensor and the PCB has been damaged, or because the PCB itself has developed a fault. When the code is active, the letter 'F' flashes on the display alongside the number 64, and the boiler locks out as a precaution to prevent any overheating damage. Because Protherm is part of the Vaillant Group, the underlying fault logic is shared across several Vaillant-platform boilers, and the F.64 description — short-circuit on an NTC sensor or defective electronics — is consistent across the range.
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
- Short-circuited or failed NTC thermistor Common
The flow or return temperature sensor (NTC thermistor) has either short-circuited or drifted so far out of specification that the PCB's analogue-to-digital converter cannot produce a valid reading. A healthy NTC typically reads several kilohms of resistance at room temperature; a short circuit will read near zero, which immediately triggers F.64. This is the single most common root cause and is usually the first component an engineer will test.
- Damaged, loose, or trapped wiring Common
The wiring loom between the NTC sensors and the PCB is routed through a relatively confined casing. Over time — or after servicing work where the casing was removed and refitted — wires can become pinched, chafed, or pull free from their connectors. Even a marginal increase in resistance on the sensor circuit, or momentary contact between two wires, is enough to confuse the ADC and lock the boiler out. Intermittent F.64 faults in particular often trace back to a partially damaged cable rather than a fully failed component.
- Faulty or degraded PCB Sometimes
If the NTC sensors and their wiring test as healthy, attention turns to the control board itself. Solder joints on the analogue input circuits can crack over years of thermal cycling, and surface-mount components near the sensor inputs can corrode, especially in boilers installed in damp utility rooms. A degraded PCB may produce sporadic F.64 codes that clear on reset before eventually becoming permanent.
- Pump or internal component fault affecting sensor readings Rare
A failing circulating pump can cause unusual temperature differentials across the heat exchanger, creating rapid swings in the flow and return temperatures that the PCB interprets as a sensor anomaly. Similarly, a partially blocked heat exchanger can produce erratic temperature readings. These are less common causes but worth considering when sensors and wiring test correctly.
How to fix it
- Check that the boiler's gas supply and electricity supply are both on DIY safe
Before anything else, confirm the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe) and that the boiler's fused spur switch is on. A brief interruption to the power supply can sometimes trigger electronic fault codes that clear themselves once supply is stable.
- Reset the boiler — up to two or three times only DIY safe
Press and hold the reset button (or follow the reset procedure in your user guide) to attempt to clear the F.64 lockout. If the boiler fires and runs normally, monitor it closely over the next day or two. If F.64 returns quickly, or if the boiler refuses to reset at all, do not keep trying — repeated resets on a genuine sensor or PCB fault will not fix the underlying problem and may mask useful diagnostic information for the engineer.
- Note any pattern to the fault DIY safe
Check whether F.64 appeared after any recent work on the boiler, after a power cut, or during cold weather. Note whether it clears on reset and how long before it returns. This information helps an engineer narrow down whether the fault is intermittent (more likely a wiring or solder-joint issue) or permanent (more likely a failed sensor or PCB).
- A Gas Safe engineer should test the NTC thermistors with a multimeter Gas Safe engineer
The engineer will disconnect the flow and return NTC sensors in turn and measure their resistance at a known temperature. A serviceable sensor reads several kilohms at room temperature; a reading of near zero ohms confirms a short circuit, while an open-circuit reading (infinite resistance) points to a broken sensor element. If either sensor is outside specification, it is replaced — this is often sufficient to clear F.64 without touching the PCB.
- A Gas Safe engineer should inspect and test the sensor wiring loom Gas Safe engineer
With the casing open, the engineer will trace both sensor cables from connector to connector, flexing the loom gently while monitoring resistance to catch intermittent breaks. Any chafed, pinched, or corroded section of cable is repaired or replaced. Connectors are reseated and checked for corrosion or spread terminals.
- A Gas Safe engineer should diagnose and, if necessary, replace the PCB Gas Safe engineer
If the sensors and wiring are healthy, the engineer will assess the control board. Minor issues such as a cracked solder joint can sometimes be repaired on-site, but in most cases a degraded Protherm/Vaillant-platform PCB is replaced entirely. Always ask for a genuine manufacturer board rather than a pattern part — aftermarket PCBs have a noticeably shorter service life. If the boiler is older and the PCB cost approaches the value of the appliance, the engineer may recommend replacement of the boiler instead.
- Call a Gas Safe registered engineer to carry out the diagnosis and repair Gas Safe engineer
Because F.64 is rooted in the boiler's electronics and sensor circuit, there is no homeowner-safe fix beyond a reset. Contact a Gas Safe registered engineer (check credentials at GasSafeRegister.co.uk) to carry out a proper resistance test of the NTC sensors, inspect the wiring, and assess the PCB. Always ask for a written quote before authorising parts.
Parts you may need
- Flow NTC thermistor (temperature sensor) · from £25
- Return NTC thermistor (temperature sensor) · from £25
- Sensor wiring harness / loom · from £35
- Replacement PCB (Protherm / Vaillant-platform) · 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 £90–£380, depending on the underlying cause.
Frequently asked questions
Is the Protherm F.64 fault dangerous?
F.64 itself is not a gas-leak or carbon monoxide fault — it is an electronics error that causes the boiler to lock out as a precaution. Your home is not in immediate danger, but you will have no heating or hot water until the fault is resolved. Do not attempt to open the boiler casing yourself; call a Gas Safe registered engineer to carry out the repair safely.
Can I fix F.64 myself by resetting the boiler?
You can try resetting the boiler once or twice — on rare occasions a power fluctuation can trigger F.64 and a reset clears it permanently. However, if the code keeps coming back, a reset alone will not fix it. The underlying cause — most commonly a failed NTC sensor or a PCB fault — needs to be diagnosed and repaired by a Gas Safe engineer. Repeatedly resetting without fixing the root cause is not advisable.
How much does it cost to fix a Protherm F.64 fault in the UK?
Most people pay between £90 and £380 all-in. If the fault turns out to be a faulty NTC thermistor — the most common cause — expect to pay roughly £90–£150 for parts and labour. If the PCB needs replacing, total costs including a genuine manufacturer board and the engineer's time typically fall in the £300–£450 range. A full PCB replacement on an older boiler that is already out of warranty can sometimes make a new boiler the more cost-effective long-term choice, so it is worth asking your engineer for an honest assessment.
Why does my Protherm F.64 fault come and go?
An intermittent F.64 that clears on reset and then returns after a few hours or days usually points to a wiring fault — a hairline crack in a cable, a loose connector, or a cold solder joint on the PCB that only loses contact when the board heats up and expands. Fully failed components tend to produce a permanent lockout. Tell your engineer about the intermittent pattern; it helps them focus on the wiring loom and solder joints rather than immediately replacing the sensor or PCB.