Er.12 Medium

Vaillant Er.12 Fault Code

Also written as ER12 — same fault.

The Vaillant Er.12 fault code means: your boiler is struggling to push water around your heating system, usually because too many radiators are turned off or there is a blockage in the pipes.. It appears on 2 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Automatic bypass
Typical repair cost £120-250 (including labour)

DIY fix possible

Ensure all radiator valves are fully open to allow water to circulate freelyCheck that your system pressure gauge is set between 1 and 1.5 barTurn the boiler off at the mains for 60 seconds and then switch it back on to reset the system

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Vaillant models that show the Er.12 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant Er.12 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026