Overview
F.64 on a Vaillant Turbomax Plus 824/828E signals an electronic/system or sensor fault. The manufacturer notes this most commonly when a flow or return (backflow) NTC thermistor has short‑circuited, or when there is an internal electronic fault on the main PCB or wiring loom. In practice this means the boiler is not getting a reliable temperature signal from the flow/return sensors or the control electronics are detecting an implausible or missing signal, and the boiler will usually lock out or go into a protected state. Severity is moderate to high for appliance operation: it will usually prevent normal heating and hot water until diagnosed and fixed, but it is not an immediate household-safety emergency like a gas escape (unless other fault symptoms exist). Because the likely causes include wiring shorts, water ingress, failing sensors, or a faulty PCB, correct diagnosis requires electrical checks and possibly component replacement. Simple reset and visual checks can be done by a homeowner, but any internal electrical work, sensor replacement, PCB replacement, or gas-related procedures should be carried out by a qualified Gas Safe engineer.
Possible Cause: Electronic system or sensor fault
Troubleshooting Steps
Safety precautions:
1) If you are not a trained engineer do not open or work inside the boiler casing. There are live electrical parts and gas components inside. Only a Gas Safe registered engineer should carry out internal repairs.
2) Before any basic checks, switch the boiler to standby and use the isolation switch or fused spur to remove mains power for simple visual checks. Do NOT isolate the gas unless instructed by an engineer.
3) If you smell gas, leave the property immediately and call the gas emergency number. Do not operate electrical switches or the boiler.
Initial homeowner checks (safe, non‑invasive):
1) Note the exact error code and any sequence of codes on the display. If the code is intermittent, write down when it appears (startup, during DHW, after running for a while).
2) Try a single reset: use the boiler reset button or switch the boiler off at the mains for 1–2 minutes then turn it back on. If the code clears but returns quickly, further diagnosis is needed.
3) Check boiler pressure on the front display. If pressure is very low (typical signs: pressure below recommended level), repressurising to the normal range may be required, but only if you know how to do this safely. Low system pressure can cause some sensor/operation issues.
4) Look for obvious external signs of problems: water leaks around or under the boiler, moisture on the PCB or connectors, scorch marks or burnt smell, or loose external wiring/cables. Take photos to show the engineer.
5) Check that external controls (room thermostat, programmers) are calling for heat and are set correctly. Ensure no external control is forcing odd temperature readings.
Diagnostic and fix steps (for a qualified engineer):
1) Enter service/engineering mode and review fault history and live sensor readings to confirm F.64 and any associated faults.
2) Visually inspect the wiring harness, sensor plugs and PCB for water ingress, corrosion, damaged insulation, bent/burnt pins, or loose connectors. Secure or replace corroded plugs as needed.
3) Test the flow and return NTC sensors with a multimeter: check continuity and measure resistance at ambient temperature and while gently warming the sensor (resistance should change with temperature). A near‑zero resistance indicates a short; infinite/very high indicates open circuit. Also check for short to earth/ground.
4) Trace and test the sensor cable harness for continuity and shorts back to the PCB. Disconnect the sensor plug and measure at the PCB connector to confirm wiring integrity. Repair or replace any damaged cable or plug.
5) If sensors and wiring test good but readings on the display are unstable, check the PCB for faults: look for loose solder joints, damaged components, or intermittent connections. Monitor live values — jumping or incorrect sensor values often indicate PCB failure.
6) Check eBUS/coding resistor/DSN settings where applicable. If a PCB has been replaced previously, ensure it is coded/programmed correctly for the boiler model (D settings/DSN) as an incorrectly configured PCB can cause electronic errors.
7) Replace the faulty component in the following order where practical: sensor(s) first (flow/return NTC), then wiring/harness, and if the fault persists replace and program the main PCB. After each replacement, clear faults and run the boiler to verify stable operation and correct sensor readings.
8) After repairs, the engineer should run full functional and safety checks: verify stable temperature readouts, check for correct burner operation and flame signal, confirm system pressure and pump operation, inspect for leaks, and perform combustion/CO checks if gas components were disturbed.
When to call a professional:
If the reset does not clear the fault, if you find any signs of water or corrosion inside the unit, if you are unable to confirm obvious external wiring issues, or in any situation that requires opening the casing or manipulating gas/electrical parts, call a Gas Safe registered engineer. F.64 often requires instrumented tests (resistance measurements, wiring continuity tests, PCB diagnostics) and possibly component replacement and programming, all of which must be performed by a qualified engineer for safety and compliance.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant Turbomax Plus 824/828E.