High
F.084 on Vaillant ecoTEC Plus 836

Your boiler is confused because it is detecting unusual temperature differences between its water pipes, which usually means a sensor is loose, incorrectly fitted, or malfunctioning.

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.

What the manual says

Manufacturer manual

Flow temperature sensor installed incorrectly. The temperature difference between the flow and return temperature sensors returns implausible values.

From this model's manual
Verified against the Vaillant Ecotec Plus installation & service manual, page 87.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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.083View →F.085View →

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