Boiler
E9
The E9 error code on a Worcester Bosch boiler indicates an overheat fault — the boiler has detected dangerously high temperatures and shut itself down as a safety measure. This is a serious fault that requires prompt attention and should not be repeatedly reset and ignored.
What does this error mean?
Worcester Bosch boilers are fitted with a safety thermostat (STL) that triggers the E9 code when the water temperature inside the boiler exceeds safe limits, typically around 95–100°C. The boiler locks out to prevent damage to components and, more importantly, to prevent a dangerous situation in your home. It will not restart normally until the root cause of the overheating is identified and resolved.
Est. repair cost
£150–£350 for pump or flow issues; £400–£700+ for heat exchanger or complex faults. Gas Safe call-out typically £80–£120.
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Check system pressure: Look at the pressure gauge on the boiler. It should read between 1.0 and 1.5 bar when cold. If it is below 1 bar, repressure the boiler using the filling loop as described in your Worcester Bosch manual.
Step 2 — Check all radiator and service valves are fully open: Ensure no valves under the boiler or on radiators have been accidentally closed, which can restrict flow and cause overheating.
Step 3 — Bleed your radiators: Trapped air reduces circulation. Use a radiator bleed key to release any air from each radiator, starting with the ground floor. Re-check pressure afterwards and top up if needed.
Step 4 — Attempt a single reset: Once you have checked pressure and bled radiators, press and hold the reset button on the boiler for 3–5 seconds. If the boiler fires up and runs normally, monitor it closely over the next few hours.
Step 5 — Do not reset repeatedly: If the E9 error returns within a short time or the boiler does not stay lit, stop resetting it. Repeated resets on an overheating boiler can cause serious damage to the heat exchanger. Call a Gas Safe registered engineer.
Step 6 — All further diagnosis and repair (pump, diverter valve, heat exchanger, STL thermostat) must be carried out by a Gas Safe registered engineer. Do not attempt to open the boiler casing or work on internal gas or water components yourself.
📞 When to call a professional
Call a Gas Safe registered engineer immediately if the E9 error returns after a single reset, if you notice any unusual smells, banging noises (kettling), or if the boiler casing feels excessively hot to the touch. Tell the engineer the exact error code (E9), your boiler model (e.g. Greenstar 25i, 30i, 8000), and whether the fault is intermittent or persistent — this will help them diagnose the issue faster.
⚖️ Repair or replace?
A pump replacement typically costs £200–£350 including labour; a heat exchanger replacement can run £400–£700 or more, which may not be economical on a boiler over 10 years old. Worcester Bosch boilers generally have a good lifespan of 12–15 years, but if yours is over 10 years old and requires a major component repair, a new A-rated boiler (typically £1,800–£2,800 installed) is worth serious consideration for long-term reliability and energy savings.