Error F.65

Safety precautions:

1) If the boiler is showing F.65, do not attempt to work inside the boiler while it is powered. Turn off the boiler using the power button and switch off the electrical supply at the isolator or fuse box before any close inspection. If you suspect a gas or major leak, turn off the gas supply and ventilate the area, and call a professional immediately.

2) Do not remove sealed covers, attempt PCB repairs, or replace gas-carrying parts yourself. These tasks require a qualified Gas Safe engineer.

Initial homeowner checks you can safely do:

1) Simple reset: press and hold the boiler power button for about 5 seconds to reset. Watch the display — if the fault does not return, monitor the boiler for a few cycles. If it returns immediately or soon after, proceed to other checks and call a pro.

2) Check ventilation and flue: ensure nothing is blocking the boiler's ventilation slots or the flue terminal outside. Poor ventilation or a blocked flue can contribute to overheating.

3) Look for visible signs of water: check under and around the boiler for leaks, corrosion, or moisture. Water ingress on wiring or the PCB area can cause electronic faults.

4) Check system temperature settings: verify that the CH flow temperature is not set excessively high (Vaillant recommends around 60–65°C for normal operation). Lowering an overly high setpoint may prevent excessive internal temperatures.

5) Check the electrical supply: confirm your fuse hasn't blown and other appliances are receiving normal power. If you have had recent power surges or repeated trips, note that this can damage electronics.

6) Note when the fault happens: only on heating, on hot water, after long running, after heavy use, or in cold weather (frozen condensate) — this information will help the engineer.

Specific diagnostic and safe fix steps (homeowner level and what the engineer will do):

1) After reset and initial checks, if the fault persists, do not open the boiler. Instead record the fault code, take a photo of the display, and note the operating conditions (CH/DHW demand, temperatures, noise, smell, visible leaks).

2) If you find water near the boiler, isolate electrical supply and gas and avoid turning the boiler back on; call an engineer. Drying visible external condensation or clearing minor external leaks may help, but internal electronic parts must be inspected professionally.

3) If ventilation or flue blockage was cleared, restart and test. If F.65 returns, schedule a Gas Safe engineer.

What a competent engineer will check and may do (for your understanding):

1) Measure input voltage and electrical supply stability; inspect the mains wiring and fusing.

2) Visually and electrically inspect wiring looms, terminals and connectors for corrosion, water damage, or loose connections; repair or replace as needed.

3) Check temperature sensors (flow, return, DHW outlet) and sensor wiring for correct readings and continuity; replace faulty sensors.

4) Check fan and flue gas temperatures to rule out high flue temperature or blocked flue causing associated overheating or safety trips.

5) Inspect the PCB and electronics for heat damage or component failure; replace PCB if faulty or if corrosion/shorts are present.

6) Test pump operation and hydraulic conditions that could cause excessively high flow temperatures.

When to call a professional:

1) If the fault returns after a reset, if you find water ingress, any burnt smells, or you are unsure, call a Gas Safe registered engineer. Electronic faults often require specialist diagnostic tools and part replacement.

2) If the boiler will not restart, or if you need the gas or electrical supply isolated, call a professional.

Additional advice:

1) Avoid repeatedly resetting the boiler to get it running long-term — a persistent F.65 usually indicates an underlying problem that needs a proper fix.

2) When you call an engineer, give them the exact fault code (F.65), describe when it occurs, and tell them what checks you already performed (reset, ventilation, visible leaks). This saves time and helps with diagnosis.

3) Keep records and photos of the fault and any visible damage; this can help with warranty or service history.

Remember: F.65 is an electronics over-temperature/protection shutdown. Initial checks are safe to do, but diagnosis and repair of electrical/PCB issues and any gas-side work must be done by a qualified, Gas Safe registered heating engineer.