d12 High

Alpha d12 Fault Code

Also written as D12 — same fault.

The Alpha d12 fault code means: your boiler is reporting an issue with the water circulation pump speed, which is preventing it from effectively moving heat around your home.. It appears on 5 Alpha models in our database.

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: Pump speed (0 = OFF, 1,2,3,4,5,6,7,8,9)
Typical repair cost £150-300 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like d12: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

Alpha models that show the d12 fault code

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

Source

Alpha d12 fault code information on this page is sourced and cross-checked against the official Alpha service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026