How to Fix Main E125 Fault Code (No Primary Circulation) UK
Main E125 means no primary circulation. UK diagnosis, parts to order, reset sequence.
What E125 means
Main E125 indicates no primary circulation. The boiler will lock out and refuse to fire until the fault is cleared.
Do not simply reset — E125 is stored in the Main error log and will re-trigger within minutes if the underlying part has failed.
Diagnostic sequence
Enter service mode on the Main boiler and read the flow/return/DHW temperatures, fan PWM, ionisation current and pump speed.
Cross-reference against the Main technical manual — E125 typically points to two or three candidate parts.
- Check wiring harness continuity to the suspect sensor before condemning it.
- Isolate at the fused spur, discharge and re-inspect connectors.
- Measure resistance / voltage at the PCB plug per Main test procedure.
Parts to have on the van
Carry a set of NTC sensors, an ignition electrode kit and a fan for Main — these three parts clear >70% of E125 call-outs.
Recommission
After the fix, run the Main boiler through a full flow test, gas rate, and combustion analysis. Log to the Benchmark book to preserve manufacturer warranty.
Frequently asked questions
- Can I reset E125 without changing parts?
- Only if the fault has cleared (e.g. low pressure repressurised). Persistent E125 means a hardware failure.
- Which is the most common cause of Main E125?
- On Main boilers, E125 most often points to the sensor or actuator matching its meaning (no primary circulation); check wiring and connector integrity first.
- Is a Gas Safe engineer required?
- Yes — any works past the boiler case seal require a Gas Safe registered engineer under UK law.
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