High
F.84 on Vaillant ecoTEC Plus 826

Your boiler is struggling because it detects an unusual temperature difference between the water leaving and returning to the unit, suggesting an issue with heat circulation or sensor accuracy.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Fault: Temperature difference at the flow and return temperature sensors

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F.22View →F.75View →

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