Potterton E125 Fault: Primary Circulation Fault — Causes & What To Do

E125 indicates a primary circuit circulation fault — the boiler can't confirm water is circulating through the heat exchanger correctly. It locks out to prevent the heat exchanger overheating. You'll usually have no heating or hot water.

The most common causes, in order of how often we see them:

• • Failed or seized pump — if the pump isn't circulating water, the boiler detects no flow and locks out. A very common E125 cause.
• • Airlock in the system — trapped air can stop circulation in part of the system, causing the flow fault.
• • Blocked or scaled heat exchanger — sludge and limescale restrict flow through the heat exchanger, so the boiler detects poor circulation.
• • Blocked system filter — a choked magnetic filter can restrict flow enough to trigger E125.
• • Frozen condensate — a blocked flue from a frozen condensate can cause overheating that presents as a circulation/overheat fault in cold weather.

You can try one reset — if it was a one-off airlock that has cleared, the boiler may run normally. But if the circulation cause remains (a failed pump, blockage, or frozen condensate), E125 will recur quickly. Don't keep resetting — repeated overheating risks heat exchanger damage. Book an engineer.

• • Check the pressure gauge — if it's below 0.6 bar, low pressure can contribute to poor circulation. Repressurise to 1.2 bar, then try one reset.
• • Bleed radiators if they're cold at the top — trapped air causes airlocks. Re-check pressure afterwards, as bleeding lowers it.
• • Check the condensate pipe outside for freezing in cold weather — thaw gently if frozen, then reset.
• • Listen for the pump — when the boiler tries to fire, you should hear the pump run. If it's silent or humming loudly, the pump may be seized and needs an engineer.
• • If E125 recurs after one reset, stop resetting — repeated overheating risks damage. Book a Gas Safe engineer.

The pump and heat exchanger sit inside the boiler casing. Replacing the pump, descaling the heat exchanger, or any work inside the casing is gas/electrical work that must be done by a Gas Safe Registered engineer. Don't keep resetting a circulation-fault boiler.

• • Test the pump — confirm it's running freely and circulating properly.
• • Check for airlocks and vent the system.
• • Inspect the heat exchanger for scale, sludge or blockage.
• • Check the condensate route and flue for blockages.
• • Inspect the magnetic filter and system flow.
• • Replace the pump or clean the heat exchanger as needed, then re-commission.

Our Gas Safe Registered (594547) engineers repair every Potterton model — Promax, Promax Ultra, Assure, Gold, Titanium and Titan — across Surrey, Berkshire, Buckinghamshire and West London, and carry common sensors, fans and ignition parts on the van. Most Potterton fault-code repairs are completed in a single visit, with any part quoted and agreed before fitting.