The boiler has sensed the primary water getting too hot and shut down on the overheat thermostat.
Most common on: Greenstar i, CDi Classic
E9 means the overheat thermostat has tripped: the primary flow temperature rose beyond a safe limit. The boiler cuts the burner to prevent damage.
Overheating is almost always a circulation problem — if the heated water is not moving away from the heat exchanger fast enough, its temperature spikes.
Top up to 1.0–1.5 bar (cold) if the gauge reads low, then reset.
Cold spots or radiators slow to heat can indicate sludge or a circulation problem worth flagging to an engineer.
Persistent E9 needs a heating engineer to check the pump, sensors and heat exchanger, and possibly a system flush.
Work on the gas side of a boiler must be carried out by a Gas Safe registered engineer. The checks above are limited to safe, non-invasive things a homeowner can look at (pressure, resets, the gas supply) — if in doubt, stop and call an engineer.
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The boiler has sensed the primary water getting too hot and shut down on the overheat thermostat. E9 means the overheat thermostat has tripped: the primary flow temperature rose beyond a safe limit. The boiler cuts the burner to prevent damage.
You can try a single reset and the safe checks below, but Worcester Bosch E9 usually needs a Gas Safe registered engineer. Never work on the gas side of a boiler yourself — it is illegal and dangerous unless you are Gas Safe registered.
A recurring E9 means the underlying cause has not been fixed — most often poor circulation from a failing pump or airlock. Resetting repeatedly will not solve it; get the boiler diagnosed properly.
Pump / circulation fault (dry fire protection)
Internal / sensor fault
See all Worcester Bosch fault codes.