Worcester Bosch E9 224 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch E9 224 fault code mean?
The E9 fault on Worcester Bosch boilers indicates that the safety temperature limiter has tripped because the central heating water reached a dangerously high temperature — typically around 105°C at the flow sensor or 130°C at the safety limiter itself. The boiler locks out deliberately to prevent further overheating and will not restart on its own. The sub-code 224 is specific to the Greenstar 8000 range and confirms the lockout was triggered by the flow temperature hitting or exceeding the limiter's cut-off point. Related E9 sub-codes you may also see include: 219 (indicating a heat exchanger fault), 220 and 221 (pointing toward a high heating temperature that could be genuine overheating or a faulty Safety Temperature Limiter rather than a true overheat event). All E9 variants result in a hard lockout requiring manual intervention before the boiler will fire again.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or seized circulation pump Common
The pump is responsible for keeping heated water moving around the system continuously. If it seizes, develops an airlock, or loses efficiency, hot water stagnates inside the heat exchanger and temperatures climb rapidly until the safety limiter trips. This is the single most common root cause of an E9 224 lockout.
- Sludge or limescale build-up restricting flow Common
Over time, magnetite sludge accumulates in radiators and the heat exchanger, and in hard-water areas limescale compounds the problem. Both reduce water flow, trapping heat inside the boiler and causing it to overheat within minutes of firing. Systems without a magnetic filter are particularly vulnerable.
- Faulty safety temperature limiter or temperature sensor Sometimes
The boiler may not be genuinely overheating at all — a degraded safety limiter or a failing flow temperature sensor can send an incorrect high-temperature reading to the PCB, which then shuts the boiler down as a precaution. E9 sub-codes 220 and 221 often suggest this scenario.
- Low system pressure reducing circulation volume Sometimes
When system pressure drops significantly below 1 bar, there is less water in circulation to absorb and carry away heat. This reduced volume can allow temperatures to build in the heat exchanger and trip the limiter, even if the pump and pipework are otherwise healthy.
- Sticking or partially blocked diverter valve Sometimes
On combi boilers, a diverter valve that is sticking or partially obstructed can impede the primary circuit flow, concentrating heat in the heat exchanger and pushing temperatures toward the E9 224 trip point. An engineer will test valve movement and check for restrictions during diagnosis.
- Fan fault causing poor flue ventilation Sometimes
A fan running slowly or intermittently disrupts the airflow needed to exhaust combustion gases efficiently. This can cause heat to build up inside the boiler casing and trigger the overtemperature limiter, even when the water circuit itself is not the primary problem.
- Blocked condensate pipe Rare
A blocked or frozen condensate pipe (common in cold weather on externally routed pipework) can disrupt normal boiler operation and in some cases contribute to abnormal temperature readings or erratic shutdowns that appear alongside an E9 fault.
- Faulty PCB Rare
A damaged or failing printed circuit board can misread sensor data or incorrectly control the pump and fan, creating conditions that lead to overheating. It can also falsely report an E9 condition when everything else is functioning normally. This is a less common but harder-to-diagnose cause.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (or follow your model's reset procedure — typically holding the flame symbol button for a few seconds) until the display clears. Allow the boiler to attempt a full heating cycle. If it locks out again immediately or within a short period, do not keep resetting — repeated resets on a genuine overheat fault risk damaging internal components that were undamaged originally.
- Check your system pressure DIY safe
Look at the pressure gauge on the boiler fascia. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, top up carefully using the filling loop (usually a braided flexible hose beneath the boiler with one or two valves). Open the valve(s) slowly, watch the gauge, and close them once the pressure reaches 1.2–1.5 bar. Do not overpressurise — stop well before 2 bar.
- Check for a frozen or blocked condensate pipe in cold weather DIY safe
Locate the external white plastic condensate pipe (usually exits through an outside wall). If the weather is below freezing and the pipe looks iced over, thaw it gently by pouring warm — not boiling — water along its length. Once thawed, attempt a single reset. If the boiler fires and stays on, monitor it. If the fault code returns, move on to calling an engineer.
- Do not attempt further resets if the fault returns DIY safe
If the boiler locks out again after one or two resets, stop. Continuing to reset a genuine overheating fault can cause heat damage to the heat exchanger, pump seals, and other internal components. The underlying cause needs diagnosing before the boiler is run again.
- Have a Gas Safe engineer inspect and diagnose the root cause Gas Safe engineer
A registered engineer will check pump operation and flow rate, inspect the heat exchanger for sludge or scale blockage, test the safety temperature limiter and flow sensors (particularly relevant if E9 220 or 221 is also present), examine the diverter valve on combi models, and verify fan and flue performance. Based on findings they will advise on a powerflush, component replacement, or further investigation.
- Arrange a powerflush if sludge or scale is identified Gas Safe engineer
If the engineer finds restricted flow caused by magnetite sludge or limescale, a powerflush — using specialist chemical cleaning equipment to force clean water through the entire system — is typically the recommended fix. A magnetic system filter should be fitted at the same time to prevent rapid recontamination.
- Replace faulty components as recommended by the engineer Gas Safe engineer
Depending on diagnosis, the engineer may need to replace the circulation pump, safety temperature limiter, flow sensor, diverter valve, fan assembly, or — in more serious cases — the heat exchanger or PCB. Only Gas Safe registered engineers should supply and fit these parts on a gas appliance.
Parts you may need
- Circulation pump (Greenstar compatible) · from £95
- Safety temperature limiter / STL sensor · from £45
- Flow temperature sensor (NTC) · from £25
- Diverter valve assembly (combi models) · from £75
- Magnetic system filter (e.g. Magnaclean Pro2) · from £85
- Fan assembly (Greenstar 8000 series) · from £130
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£450, depending on the underlying cause.
Frequently asked questions
What is the difference between E9 219, E9 220, E9 221, and E9 224?
All four are sub-codes that appear alongside the main E9 overtemperature fault. Code 219 specifically indicates a heat exchanger fault — often a cracked or heavily scaled exchanger. Codes 220 and 221 suggest the boiler has detected a high heating temperature but point toward the Safety Temperature Limiter itself being faulty, rather than a genuine overheat event. Code 224 (the focus of this page) confirms the safety limiter actually tripped due to flow temperature exceeding the cut-off threshold on the Greenstar 8000. Telling your engineer which sub-code is displayed helps them narrow down the diagnosis before they even open the boiler.
Can I keep resetting the boiler to get heat while I wait for an engineer?
It is not advisable. One reset to see whether the fault clears is reasonable, but if the boiler locks out again it is trying to tell you something is genuinely wrong. Running an overheating boiler repeatedly can damage the heat exchanger, pump seals, and internal seals — turning what might be a straightforward pump or sludge job into a much more expensive repair. Leave it off and get an engineer out promptly.
How much does it typically cost to fix an E9 224 fault in the UK?
Most people with this fault pay somewhere between £150 and £450 all-in. A limiter or sensor swap at the cheaper end, or a pump replacement with labour, tends to sit in that range. A full powerflush to clear sludge usually runs £300–£600 depending on the size of the system and your location. For more serious cases — a replacement heat exchanger (£400–£650 including labour) or a new PCB (£450–£800) — costs climb considerably higher. If your boiler is over 10 years old and facing one of those bigger repairs, it is worth getting a replacement quote at the same time.
Will an annual boiler service prevent the E9 224 fault from coming back?
It significantly reduces the risk. A yearly service gives an engineer the chance to catch a tiring pump, early sludge build-up, and a deteriorating sensor before any of them cause a lockout. Fitting a magnetic filter (if you do not already have one) and using a system inhibitor chemical also help prevent sludge reaccumulation. Worcester Bosch offer their own service plan which includes annual servicing and covers repairs up to £1,500 if a fault is found — worth considering if you want predictable running costs.