High
125 on Potterton Assure 25 Combi

Your boiler is locking out because water is not flowing properly through the heating system inside the boiler.

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

Primary water temperature has not changed by 1° C between 15 and 30 seconds of burner lighting, or primary water temperature actual temperature twice exceeds the selected temperature by 30° within 10 minutes of burner lighting (poor primary circulation)

From this model's manual
Verified against the Potterton/Potterton Bm Potterton Assure Combi Installation Pdf installation & service 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 125: 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

110View →119View →133View →

All 20 fault codes on the Assure 25 Combi

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

0915202840103104107109110118128130133134135160321384

Related guides you might find useful

Banging, kettling or whistling noise

Each noise points to a different fault.

Read →

No hot water but heating works

Almost always a stuck diverter valve on a combi.

Read →

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →