Error F.65

Safety first

1) If the boiler is showing F.65 repeatedly, do not attempt internal electrical repairs. Turn the boiler off using the mains isolator or the external switch and leave it off until a qualified engineer attends if the fault persists.

2) If you smell burning or see smoke or obvious fire risk, isolate the mains and call the emergency services immediately.

3) Do not remove the boiler cover or touch wiring if you are not qualified. Mains voltages and gas components can be lethal.

Initial homeowner checks (safe, non-invasive)

1) Reset the boiler: many Vaillant boilers allow a single reset via the control panel. Try one reset and observe. If the fault returns immediately or repeatedly, do not continue resetting.

2) Allow the boiler to cool for 20–30 minutes and then restart to see if the fault clears temporarily. Note whether the fault returns under normal load (when heating or hot water is requested).

3) Check the area around the boiler: ensure there is adequate ventilation and no external heat source (hot water cylinder, heat lamp, direct sunlight or a nearby appliance) heating the boiler casing or control area.

4) Look for visible signs from outside the casing: condensation/water leaks around the boiler, damp patches on the wall, or recent plumbing leaks that could have dripped onto the electronics. Do not open the casing if you find moisture; isolate power and call an engineer.

5) Check external flue/venting: ensure the flue terminal is not blocked by debris or bird nests. A blocked flue can raise temperatures inside the unit and trigger safety shutdowns.

6) Check the circuit protection: verify the boiler’s fuse or fuse in the fused spur (if accessible) and the consumer unit for tripped RCD/MCB. Do not open the boiler to access internal fuses; only check accessible house fuses/trips.

Information to collect for the engineer

1) Note exactly when the fault started, whether it happens on demand or intermittently, and whether anything changed recently (power cuts, storms, plumbing/electrical works, overheating nearby appliances).

2) Record the full model and serial number and any other fault history shown on the display. Some Vaillant units store a fault history (on some models press and hold the I and - buttons) — tell the engineer what you see but only attempt this if it is just button-pressing without opening the casing.

3) Note any visible signs you observed: damp, burning smell, frequent power trips, whether the boiler cools then runs briefly before faulting again.

What an engineer will likely check and do (for your information)

1) Visual inspection of PCB and wiring for burn marks, corrosion or water ingress. Loose or corroded connectors can raise resistance and heat.

2) Measure input mains voltage and ripple; an unstable mains supply can stress electronics.

3) Check NTC sensors and temperature-sensing circuitry to ensure the temperature reading is valid and hasn’t been caused by a defective sensor.

4) Check for elevated flue or internal temperatures that could transfer heat to the electronics and inspect flue and fan operation.

5) Run diagnostic tests and review stored fault history. If the PCB is faulty or sensors/wiring are damaged, the engineer may replace the defective component(s).

When to call a professional

1) If the fault returns after a reset, if you find moisture, smell burning, or if the boiler repeatedly enters F.65, switch the boiler off and call a Gas Safe registered engineer. Repairs involving the PCB, wiring, gas valve, or internal sensors must be done by a qualified engineer.

2) Do not attempt PCB replacement, internal wiring repairs, or mains electrical testing yourself. These are live-electrical and gas-risk tasks that require certification and safe isolation procedures.

Interim measures while waiting for a Gas Safe engineer

1) Turn the boiler off and isolate power if the fault is persistent. Leave it off to prevent further damage.

2) Keep the area dry and well ventilated and remove any nearby heat sources that could be heating the boiler casing.

3) Provide the engineer with the information you collected to speed diagnosis.

Final note

F.65 is an electronics over-temperature/protection fault. Simple resets can sometimes clear a one-off event, but recurrent faults usually indicate failing electronics, wiring issues or external environmental problems and require a Gas Safe engineer for safe, correct diagnosis and repair.