Overview
E125 on a Baxi 600 Combi means the boiler has detected a circulation fault on the primary heating circuit. In plain terms the boiler is not sensing proper movement of heated water through the system. Common causes are low system pressure, trapped air (airlocks), a blocked flow path from sludge or limescale, a failing circulation pump, or a component such as the pressure relief valve losing water. Less commonly it can be an electrical fault that prevents the pump from running. Severity is moderate. The boiler will often lock out to protect itself, so you will lose heating and possibly hot water. It is not usually an immediate gas safety emergency, but leaving a circulation fault unaddressed can cause further damage to the pump, heat exchanger, or components and will leave you without heating. Some basic checks can be done safely by the homeowner, like checking pressure, bleeding radiators, and a controlled repressurisation. Any work that requires removing covers, working on the pump, replacing valves, powerflushing, or accessing gas/electrical internals must be done by a Gas Safe registered engineer.
Possible Cause: Circulation fault (primary).
Troubleshooting Steps
Safety first
- Before you do anything switch the boiler off and isolate electrical power at the fuse or fused spur and allow the boiler to cool. Do not remove the boiler casing or attempt internal repairs unless you are a qualified Gas Safe engineer. If you smell gas, leave the property, ventilate, and call your gas emergency number immediately.
Initial homeowner checks
1) Check the boiler pressure gauge. Typical boilers run around 1.0 to 1.5 bar when cold. If the pressure is below about 1.0 bar top up the system using the filling loop following the boiler user manual. Close the filling valves when the correct pressure is reached and secure the filling loop if it is removable.
2) Look for obvious leaks around the boiler, pipework, radiators and valves. If you find a leak isolate the water supply and call a Gas Safe engineer.
3) Bleed radiators to release trapped air. Start with the radiator closest to the boiler and work around the house. Re-check system pressure after bleeding and repressurise if needed.
4) Reset the boiler after carrying out the above checks. Use the reset procedure in the manual. If the E125 clears and the boiler runs normally monitor the pressure over the next days to ensure it stays stable. A repeating drop in pressure indicates a leak or PRV issue and needs an engineer.
Diagnostic signs to observe (do not remove covers)
- Listen at the boiler: if the pump is trying to run you may hear a hum or vibration. A noisy knocking or gurgling can indicate air or poor flow. A silent pump when the boiler is calling for heat suggests an electrical or pump failure.
- Feel the flow and return pipes (carefully, after turning the boiler on briefly): the flow pipe should be noticeably warmer than the return if circulation is working. If both are similar cold or lukewarm there is little or no circulation.
When to stop and call a professional
- If topping up pressure, bleeding radiators and resetting do not clear E125
- If you find a leak, a dripping PRV, or the pressure keeps falling
- If the pump is silent or humming but there is no circulation, or if you suspect a blockage, sludge or a blocked heat exchanger
- If the fault recurs or there are electrical faults or multiple error codes
Professional repairs an engineer will perform
- Manual testing and electrical checks of the pump and control wiring, replacement of a failed pump if required
- Powerflush or chemical flush to remove sludge and restore flow where blockages exist
- Inspection and replacement of leaking PRV, autovents or faulty components
- Check and test the heat exchanger and other internal parts; any removal of casing or work on gas components must be done by a Gas Safe registered engineer
Final note
If E125 persists after the basic homeowner checks call a Gas Safe registered heating engineer. Avoid attempting internal repairs, pumping through the system yourself, or dismantling gas-bearing components. A qualified engineer will diagnose the root cause and carry out safe, compliant repairs.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 600 Combi Gas Boiler.