Error F.84

Safety precautions:

- If you are not competent with gas or electrical appliances, do not open the boiler casing or attempt internal repairs. Contact a Gas Safe engineer.

- Before doing any visual checks, turn the boiler off at the programmer and the mains electrical isolator. If you need to isolate the gas supply, only do so if you are trained — otherwise wait for an engineer.

- Avoid touching live electronics or the gas components. Do not bypass safety devices or attempt to operate the boiler if it is locked out for safety.

Initial homeowner checks you can do (safe, no internal access):

1) Note the circumstances: when the F.84 appeared, any recent work on radiators, system bleeding or refilling, or loss of power.

2) Try a controlled reset: switch the boiler off at the programmer and the mains isolator for 30–60 seconds, then switch back on. If the code clears temporarily but returns, that indicates an ongoing fault.

3) Check system pressure on the boiler gauge. For most combi/system boilers the cold pressure should be around 1–1.5 bar. If pressure is very low, top up using the filling loop per the boiler handbook. Do NOT continue if you see active leaks.

4) Check for visible leaks or water around the boiler, pipes and unions. If you find leaks, turn the boiler off and call an engineer.

5) If you or a tradesperson recently bled radiators or refilled the system, bleed radiators to remove trapped air as airlocks can cause strange sensor differentials. Re-check pressure after bleeding.

If you are mechanically confident and the boiler is isolated electrically (only do this if competent):

1) Remove the outer casing only after isolating the electrical supply. Visually inspect the flow and return sensor connectors (small plug-in leads). Look for loose plugs, corroded pins, signs of water ingress or damaged wiring. If anything is visibly loose, re-seat connectors firmly and replace the casing before powering up and testing. If you see water on the PCB or severe corrosion, power down and call an engineer.

2) Do not pull sensors out of position to measure unless you have the correct tools and knowledge. If you can access the NTC plug and have a multimeter and the service knowledge, an engineer will measure resistance: typical NTC values are around 12 kΩ at ~20 °C (manufacturer spec varies). Incorrect or open-circuit readings indicate faulty sensors or wiring.

What a professional will do (use this to brief the engineer):

- Use Vaillant diagnostic software or multimeter to read flow and return NTC resistances and live temperature values to identify which sensor is implausible, inverted, shorted or open.

- Check wiring harness and plugs for continuity, shorts, water damage and secure connections on the PCB.

- Confirm sensors are fitted to the correct pipes and correctly positioned in the flow/return.

- Inspect pump operation, system flow rate and for airlocks or blocked valves that might cause abnormal temperature differences.

- Inspect heat exchanger for signs of limescale damage or leaks that could allow water into sensor areas or electronics and cause F.84. If a heat exchanger is leaking or heavily scaled it may need repair or replacement.

- Replace defective thermistors and/or wiring as required, then test system operation and clear the fault with the correct diagnostic procedure.

Final notes and when to call a professional:

- Because F.84 is a sensor/measurement safety fault and can be caused by wiring, sensor placement, electronic or heat exchanger faults, most permanent fixes require a Gas Safe registered engineer. Do not attempt gas-side or PCB repairs yourself.

- If a simple reset, repressurising, bleeding radiators or re-seating an easily accessible connector does not permanently clear the fault, contact a Gas Safe engineer. When you call, tell them the exact code (F.84), what you have already tried, the boiler pressure reading, and any recent work or changes to the heating system.