High
d6 on Worcester Bosch Highflow 250

Your boiler has detected that the internal water storage tank is overfilling, which forces the system to stop working to prevent internal flooding or water damage.

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

Internal heat bank overflow

Shared across the range
This code is shared by the boilers in this range, which use the same control board. It may not be printed in this exact variant’s manual.
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.

Find a Gas Safe engineer
Search the official register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like d6: 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.

Related fault codes

d1View →

All 57 fault codes on the Highflow 250

Seeing a different code? Every code this model can display, in one place.

6A6C6d9UA1A4A5A7A8ACAdB1b2b3b4b5b6b7C1C4C6D1d3d5E2E5E9EAF0F1F7FAFdH1H4H7H9HCHeatbank Overheat StatHot water high limit statNo codeoverheat lock-out statePOINT BPOINT CPOINT DPOINT EPOINT FPOINT GPOINT HPOINT IPOINT JPOINT KPOINT NS9Safety limit statSy

Related guides you might find useful

Boiler keeps losing pressure

Six most common causes of dropping pressure.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →