Error F.78

Safety first

1) If you are not competent with electrical or gas appliances, stop and call a Gas Safe / certified Vaillant engineer. Working on a boiler can be dangerous. 2) Before doing any checks switch the boiler to the OFF position and isolate mains power at the external switch or fuse spur. Allow the boiler to cool. 3) Do not open live PCB covers or intervene with gas components. If you do any electrical testing, ensure you are trained and use correct insulated tools.

Initial homeowner checks you can do (no tools required)

1) Note any other fault codes displayed (F.91 is commonly seen with related cylinder sensor faults). 2) Try a simple reset: switch the boiler off at the front panel and at the external isolator, wait 30–60 seconds and turn back on. If the F.78 does not return, monitor for a while; repeated lockouts indicate an underlying fault. 3) Inspect visible wiring and connectors: check the external controller/link box and the actoSTOR if present for loose plugs, visibly disconnected wires, signs of corrosion, water ingress or physical damage. Make sure connectors are seated firmly (do not force connectors you cannot access safely).

More technical checks (only if you are competent and comfortable with low-voltage electrical checks)

1) Check the UK link box / external controller connections where the hot water NTC is bridged. Ensure the bridging link or plug is present and correctly fitted. 2) Visually inspect the NTC plug and wiring back to the cylinder/actoSTOR and boiler for kinks, chafing, rodent damage or moisture. 3) If you have a multimeter and understand sensor testing, you can check the wiring continuity between the sensor plug and the boiler/link box to confirm there is no open circuit, and measure the sensor resistance. An open circuit (infinite resistance) or a direct short to earth indicates a fault in the cable or sensor. Note: exact resistance values vary by sensor type and temperature—if unsure, stop and call an engineer.

If fault persists or for any repairs

1) If a loose connector or temporary issue fixed the code after a reset, still arrange a service if you find corrosion or intermittent contacts—the fault can return. 2) For defective sensor, cable, plug or actuator electronics replacement, and for any continuity/resistance testing beyond basic checks, call a Gas Safe / certified Vaillant engineer. They will have the correct diagnostic tools, manufacturer data (resistance/temperature charts), genuine replacement NTCs, harnesses, link boxes or actoSTOR parts, and the authority to work on gas appliances. 3) Typical professional fixes include reseating or replacing the NTC plug, replacing a damaged wiring harness, fitting a new DHW outlet NTC, repairing or replacing the external link box/actoSTOR electronics, or investigating PCB/eBUS communication faults.

Other important notes

1) Do not repeatedly reset the boiler if the F.78 returns immediately; this can mask a real safety or control issue. 2) Keep records and photos of any loose connectors or damaged cables you find—this helps the engineer diagnose faster. 3) Use only qualified engineers and genuine parts for replacements. If you are in the UK, ensure the engineer is Gas Safe registered and experienced with Vaillant systems.