Overview
The E9 277 fault on a Worcester Bosch GB162 means the boiler's safety temperature sensor has detected a flow temperature above about 95°C and the boiler has shut down as a safety response. This is an overheat indication from the central heating flow sensor (or the safety temperature limiter) rather than a simple ignition or pressure fault. The boiler is protecting itself from damage or unsafe operation by stopping the burner. Common reasons for this reading include poor or no circulation (blocked or seized pump, closed or stuck valves, airlocks), severe limescale or sludge in the heat exchanger reducing flow and causing local overheating, a failed temperature sensor or wiring fault, or a faulty PCB reporting incorrect values. Because the fault is an overtemperature condition it must be treated seriously — continued operation can risk damage to boiler components and create scalding risk. Some initial checks and a single controlled reset are reasonable for a homeowner, but diagnosis and repairs that involve the gas, sealed components, replacement of safety limiters, sensors, pump or PCB must be done by a qualified Gas Safe registered engineer. If the fault persists after safe basic checks, turn the boiler off and book an engineer immediately.
Possible Cause: The safety temperature sensor has measured temperature in excess of 95°c.
Troubleshooting Steps
Safety precautions:
1. If you smell gas or suspect a gas leak, leave the property immediately and call the gas emergency number. Do not operate electrical switches.
2. If the boiler shows E9 277, avoid repeatedly running the appliance. Turn the boiler off at the programmer and at the main isolator. Allow the boiler to cool before any checks.
3. Do not open the boiler casing or attempt to work on gas, fuel or combustion parts unless you are a Gas Safe engineer. Electrical isolation before touching any wiring is essential.
Initial homeowner checks you can safely do:
1. Note and photograph the display so you have the exact code (E9 277) and any additional cause code. This helps the engineer.
2. After isolating and allowing the boiler to cool for 10–30 minutes, visually inspect for obvious leaks, scorch marks, burning smells or signs of water around the boiler. If present, keep the boiler off and call an engineer.
3. Check the central heating system pressure on the boiler gauge. Typical cold system pressure is about 1–1.5 bar. Very low pressure can cause pump issues; very high pressure is also a problem. Refill the system only by following the boiler manual instructions (use the filling loop); do not improvise.
4. Check room thermostat and programmer settings to ensure they are demanding heat. If the boiler was working hard for a long time, let it cool and try a single reset as described below.
5. Listen for the pump when the boiler is running (after a controlled restart). A humming or running noise is normal; if the pump is silent and the burner tries to run, that suggests a circulation problem. Do not remove covers to access the pump unless you are qualified.
Safe reset and observation (one attempt only):
1. If there is no smell of gas, no visible damage and you have completed the basic checks, you can try one controlled reset. Follow your boiler manual: either press the reset button for three seconds or hold the temperature dial at the reset position for three seconds, then restore normal settings.
2. Turn the boiler back on and carefully observe. If the boiler clears the fault and runs normally, monitor radiators and system behaviour for the next hours—note any unusual noises, uneven heating, or recurrence of the E9 code.
3. Do not keep resetting the boiler more than once. If the E9 277 returns, switch the boiler off and contact an engineer.
Further diagnostic checks to report to the engineer (do not attempt complex repairs yourself):
1. Note whether certain radiators are cold while the boiler reports heat demand — this suggests circulation problems, trapped air or radiator valves closed.
2. Note any recent work on the heating system, new radiators, powerflushing or additions, or symptoms such as banging or gurgling noises.
3. If you live in a hard-water area or the boiler is old with no system protection, mention possible limescale or sludge build-up to the engineer.
When to call a professional and what they will likely do:
1. Call a Gas Safe registered heating engineer immediately if the fault persists after a safe reset, if you see leaks, smell burning or gas, or if the boiler repeatedly overheats.
2. The engineer will correctly isolate and test sensors, measure flow and return temperatures, check pump operation and impeller condition, inspect and test the safety temperature limiter and flow sensor wiring, assess the heat exchanger for limescale or blockage, and check the PCB for faults. Replacement of the safety limiter, pump, sensors, heat exchanger repairs or PCB replacement are specialist tasks and must be carried out by a qualified engineer.
What to tell the engineer and how to prepare:
1. Provide the exact model (Worcester Bosch GB162), error code E9 277, any additional cause code shown, age of boiler, last service, and a description of symptoms (when it started, noises, radiator behaviour, any recent messing with system settings).
2. Keep the boiler powered off until the engineer instructs otherwise. If possible, mark or photograph thermostatic radiator valve positions and system pressure for the engineer.
Final safety note:
E9 277 indicates an overtemperature situation. It is a protective shutdown but also a signal of potentially serious internal faults. After the basic safe checks and a single reset attempt, do not attempt internal repairs yourself. Arrange for a Gas Safe registered engineer to diagnose and repair the underlying cause.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.