What F64 means
F64 is the boiler's way of saying that a signal it relies on for safety cannot be trusted. It is not telling you the water is too hot or the pressure is too low. It is telling you the boiler's ability to measure those things has failed somewhere between the sensor and the board.
Because the signal is safety-relevant, the boiler locks out rather than carry on with a reading it cannot verify. The flow sensor helps protect the boiler from overheating and the flame-sensing electrode confirms there is a flame, so neither can be ignored. Vaillant's wording varies slightly between manuals, which helps show what the boiler is actually checking:
| Vaillant document | Models | F64 wording |
|---|---|---|
| ecoTEC plus installation and maintenance instructions, 0020244997 | VU and VUW ../5-5, combi 825 to 838 | "Fault: Electronics/sensor" |
| ecoTEC pro installation and maintenance instructions, 0020244995 | VUW 246/5-3 and 286/5-3 | Electronics or sensor fault |
| ecoTEC plus open-vent instructions, 0020238195 (2021) | VU ../6-5 OVZ | "Fault: Electronics / sensor / analogue-to-digital converter" |
| ecoTEC plus LPG instructions, 0020239015 | VU 246/6-5 OVZ (P-GB) | "Fault: Electronics/NTC" |
The analogue-to-digital converter in the 2021 wording is the part of the board that turns a sensor's electrical signal into a number the boiler can read. So F64 covers the whole chain: the sensor, the cable and the board that interprets it.
Common causes of F64
A flow or return sensor that has short-circuited
The 2021 open-vent manual names "Flow or return NTC short circuited" as a cause. NTC sensors are the temperature sensors clipped to the pipes going out of and coming back into the boiler. A sensor that has failed in this way gives a reading the board cannot accept.
A defective safety-relevant sensor or cable
The ecoTEC plus ../5-5 manual widens this to any safety-relevant sensor or its cable. Vaillant's checklist for engineers includes the flow sensor, the cable to it, and the flame-sensing electrode (for example the ionisation electrode), which can give an unstable signal.
Defective electronics
Every version of the table lists the electronics. If the sensors and cables test correctly, the board is the remaining suspect.
How F64 differs from F00 and F01
F00 and F01 point at a named sensor: the flow temperature sensor and the return temperature sensor respectively. F64 is broader. It tells you that a safety-relevant signal does not add up, without saying which one. That is why F64 often takes longer to diagnose. If your boiler has shown F00 or F01 before F64, write that down, because it points the engineer straight at the likely sensor.
Safe checks before you call an engineer
- Note when F64 appeared. Was it during heating, during hot water, at start-up, or after a power cut or thunderstorm?
- Look under the boiler, without removing any covers, for drips or damp. Water reaching the wiring or electronics can cause sensor and board faults.
- Check the fused spur is on and that the rest of the house has normal power.
- Switch the boiler off at the fused spur for about a minute, then back on, and see whether the boiler starts normally.
- If F64 is still showing, press reset as your manual describes. Stop after two or three attempts in total.
- If the boiler runs for a while and then shows F64 again, note roughly how long it ran. That pattern helps separate an intermittent cable fault from a sensor that has failed outright.
If F64 clears after a power cycle and stays away, keep a note of the date anyway. A second appearance suggests a real fault rather than a one-off glitch, and that is the point to book an engineer.
Do not remove the casing or unplug sensors to "test" them. The sensors sit next to hot pipes and live electrics, and a wrongly refitted sensor can stop the boiler protecting itself from overheating. Leave all of this to a Gas Safe registered engineer.
What the engineer will check
Following Vaillant's checklist, the engineer will test the flow temperature sensor and its cable, then the return sensor, looking for a short circuit. They will check the flame-sensing electrode for an unstable signal and look at the connections and cable harness. Only once those pass will they condemn the board. Expect them to read the boiler's fault history too, as earlier sensor codes help confirm the culprit. On the LPG ecoTEC plus, where Vaillant words F64 as an electronics or NTC fault, the temperature sensors are the natural starting point.
Replacing the board on the first visit without testing the sensors is worth questioning, since a short-circuited sensor is one of the causes Vaillant names and is a cheaper part.
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Flow or return temperature sensor (NTC) | Typically £100 to £200 |
| Cable, plug or wiring repair | Typically £100 to £180 |
| Ignition or flame-sensing electrode | Typically £90 to £180 |
| PCB replacement, fitted | Typically £250 to £550 |
If a new board is needed, the engineer must set it up for your boiler, otherwise you may see F70, the invalid DSN code. Our Vaillant fault codes hub lists other ecoTEC sensor and electronics codes.
What to tell the engineer
Give the full code, any other codes the boiler has shown in recent weeks, whether it followed a power cut, storm or leak, and whether the boiler restarts after a reset and for how long. Mention any recent work on the boiler, particularly if a sensor was replaced, as a sensor that was not refitted properly can cause similar faults. If the boiler is still under guarantee, contact Vaillant or your installer before booking anyone else.