Overview
The E9 276 error on a Worcester Bosch Greenstar i boiler means the boiler has detected the primary flow temperature has exceeded its safety threshold (reported as above ~95°C) and the safety temperature limiter has tripped. In short, the boiler has overheated or the temperature sensor/limit circuit is telling the control board that the flow temperature is dangerously high, so the appliance has shut down to protect itself and your home. This can be caused by poor circulation (blocked or failed pump, stuck closed valves, airlocks), restricted flow through the heat exchanger (limescale, sludge or debris), a faulty temperature sensor or safety limiter, or less commonly an electrical/PCB fault giving incorrect readings. Severity ranges from moderate to high: it is a safety shutdown to avoid damage and fire risk, so you should not ignore it. Some basic checks are safe for a homeowner, but diagnosing and repairing pumps, heat exchangers, sensors or replacing safety devices and working on gas components must be done by a Gas Safe registered engineer. If a simple reset and basic checks (system pressure, visible leaks, thawing a frozen condensate pipe) clear the code temporarily, you still need a professional inspection to find the root cause. Repeated resets are not recommended; if the fault returns after a reset, stop using the boiler and call a qualified engineer.
Possible Cause: Primary flow temperature exceeded 95°c.
Troubleshooting Steps
Safety precautions:
1. If the boiler is showing the E9 276 error repeatedly, do not continue to force it to run. The code indicates an overheat condition; continued operation risks damage.
2. Only carry out visual and non-intrusive checks. Any work on gas, the burner, wiring, internal components, or replacing parts must be done by a Gas Safe registered engineer.
3. Before touching pipework, be aware it may be hot. Switch off electrical power to the boiler at the isolator and turn off the thermostat/room controls if you need to leave the appliance off.
Initial homeowner checks you can safely do:
1. Note the exact error code and any additional cause number shown on the display. Write it down for the engineer.
2. Try a single reset: follow the boiler’s normal reset procedure (use the quick-info/reset button per the manual). If the boiler clears and runs normally, monitor closely. If the E9 returns, stop resetting and call an engineer.
3. Check boiler water pressure: on a sealed system the gauge should be around 1.0–1.5 bar when cold. If it is below 1.0 bar, top up the system following Worcester’s instructions for your model. Low pressure can affect circulation. If you are unsure how to top up, stop and call an engineer.
4. Look for visible leaks around the boiler, radiators and pipework. A leak that is dropping system pressure or causing the pump to cavitate can cause overheating.
5. Listen to the pump when the boiler tries to run: you may hear a low hum. If the pump is silent when it should be running, or it sounds noisy/grinding, that suggests a pump fault or blockage. Do not try to remove the pump or open its casing — call an engineer.
6. Bleed radiators to remove trapped air which can prevent proper circulation. After bleeding, re-check pressure and top up if needed.
7. Check the condensate pipe (if fitted) for freezing or blockage in cold weather and thaw with warm (not boiling) water if you can safely access it. A blocked condensate won’t normally cause this exact code but is a common simple fault to check.
Specific diagnostic steps and homeowner actions (safe, non-invasive):
1. Record conditions when the fault occurred: was the heating on full, just starting, or after a long run? Was domestic hot water being used? This helps the engineer identify whether the problem is circulation under load or a sensor/PCB fault.
2. If the boiler has a "quick info" or service menu, check for additional fault cause codes or warnings and write them down.
3. After topping up pressure and bleeding radiators, perform one reset. If the boiler runs and the flow/return pipes (outside the boiler) both get hot and the heating distributes normally, the immediate issue may be resolved, but you should still book a service if the boiler hasn’t been serviced within 12 months.
4. If the boiler still locks out with E9 276, and you can detect a significant temperature difference between flow and return pipes (flow very hot, return much cooler) this points to poor circulation (pump, closed valves, blockage). If flow and return are both very hot and no circulation is occurring, that points to pump failure or blockage in the heat exchanger.
When to call a professional (and why):
1. Call a Gas Safe registered engineer if the reset does not clear the code, the code returns after a short time, you detect pump noise or failure, or you suspect scale/sludge in the heat exchanger. The likely causes that require a professional are: failed pump, seized or blocked diverter/3-port valve, limescale or sludge blocking the heat exchanger, a faulty safety temperature limiter or flow sensor, or PCB faults.
2. Do not attempt to bypass safety devices or remove internal panels unless you are qualified. Replacing sensors, limiters, the pump, or the heat exchanger involves gas and electrical isolation, and potentially draining the system; these are not DIY tasks.
3. When you contact an engineer, provide the exact model, serial number, the full fault code (E9 276) and any additional cause numbers, what you’ve already tried (pressure top-up, bleed, reset), and when the fault started. If the boiler is under warranty or service contract, mention that.
Final notes:
1. The E9 276 is a safety shutdown — do not repeatedly reset and run the boiler if the fault persists. Leave the appliance off and get professional help.
2. Regular servicing and system treatment (powerflushing for sludge, scale inhibitors for hard water) reduce the chance of circulation and heat exchanger problems that can lead to overheating.
3. Keep records of faults and servicing to help with diagnosis and any warranty or insurance claims.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch Greenstar I System / Combi Boiler.