Error F.64

Safety precautions:

1) If the boiler is displaying a fault, put it into the OFF position and isolate the electrical supply at the fused spur or consumer unit before doing any internal inspection. Never work on the boiler with power applied. 2) Do not attempt to work on gas valves, ignition components or any part of the fuel system unless you are a Gas Safe / certified engineer. 3) When the PCB has been powered you must allow capacitors in the switching power supply to discharge. Leave the PCB unpowered for at least 10 minutes and, if you are going to touch the board, verify voltage on the large supply capacitor with a suitable DMM to ensure it has discharged. 4) If you are not confident with electrical safety, PCB inspection or using a multimeter, stop and call a qualified engineer.

Initial homeowner checks (safe, non-invasive):

1) Note the exact error code, whether it is persistent or intermittent, and any pattern (e.g. happens when heating calls for hot water or during running). Take a photo of the display. 2) Try a basic reset: follow the boiler reset procedure (use the display controls; press the reset/menu sequence per your manual) and see if the fault reoccurs. If it is one-off and does not return, monitor the boiler closely. 3) Check the boiler’s external power supply and any RCD/fuse to ensure the boiler is receiving stable mains power. 4) Check boiler water pressure and top up if low (if you know how to do so safely) — low pressure can cause unrelated faults but is quick to check. 5) Look for visible signs at the exterior: condensation or water ingress around the electronics cover, burn smells, discoloured connectors or obvious loose wiring. 6) Check the condensate trap and discharge pipe for blockages if accessible — a blocked condensate can cause faults and shutdown.

Safe visual PCB and connector checks (only if isolated and you are confident with basic safety):

1) Isolate mains and wait 10+ minutes. Remove the electronics cover only if the boiler is dead and isolated. 2) Visually inspect the PCB for bulging or leaking electrolytic capacitors, scorch marks, cracked solder joints or corroded connectors. On older ecoTEC Plus boards, capacitors C5/C9 (often 470µF/35V) are known failure points that can cause intermittent electronics errors. 3) Check and reseat any sensor and harness connectors that are accessible and intended to be serviceable. Take photos before disconnecting so you can reconnect correctly.

Basic diagnostic steps for a competent person with a multimeter (ONLY if competent and safe to do so):

1) With the boiler isolated, you can check continuity of accessible sensor wiring and for obvious open circuits. 2) For temperature sensors (NTCs) you may measure resistance and compare with the values in the service manual or wiring chart (note: NTC resistance varies with temperature; only use if you have the correct reference values). Do not attempt to measure anything on live mains. 3) If you find a sensor that reads open circuit or its resistance is implausible for the current temperature, it is a candidate for replacement. 4) If the PCB shows obvious failed capacitors (bulging, leaking), this can cause the sorts of intermittent electronics/NTC faults reported. Replacing electrolytic capacitors on the PCB can cure the issue, but this is an electronics repair requiring soldering skills, correct component selection (value, voltage, low ESR recommended) and safety precautions around stored high voltages.

What to avoid doing yourself:

1) Do not power the PCB and probe on-board voltages or attempt repairs if you are not trained in electronics. High voltages can remain on SMPS capacitors. 2) Do not attempt gas valve, burner, ignition or flue work unless you are a registered gas engineer. 3) Do not throw away or replace the PCB without first discussing diagnostics with a qualified engineer — many boards can be repaired by replacing a small number of capacitors or connectors.

When to call a professional and what information to give them:

1) Call a Gas Safe / qualified Vaillant engineer if the fault returns after reset, the boiler is locked out, you find suspect components (bulging capacitors, scorched board) or you are unsure at any step. 2) Provide the engineer with the model, serial number, exact fault code (F.64), frequency (intermittent or constant), age of the boiler, what you have tried, and any photos you have taken of the PCB, connectors or symptoms. 3) Ask the engineer to check sensors, wiring harness, and the PCB. For ecoTEC Plus boilers older than 7–8 years, discuss the possibility of PCB capacitor failure and whether repair (capacitor replacement) is an appropriate alternative to full PCB replacement.

Final note: F.64 is a safety-related electronics/sensor fault. While a homeowner can perform non-invasive checks and a reset, accurate diagnosis and safe repair usually require a qualified engineer. Do not attempt gas or extensive electrical work on the boiler yourself.