Overview
E9 285 on a Worcester Bosch GB162 means the boiler's return temperature sensor has detected a temperature above about 95°C and the boiler has gone into an overheat safety shutdown. The boiler uses this sensor to monitor the temperature of the water returning from the central heating system; if that temperature gets too high the control logic will stop the appliance to protect the heat exchanger and other components. This fault is commonly caused by poor circulation or heat transfer in the heating circuit: a blocked or seized pump, closed or stuck valves, heavy sludge or limescale in the heat exchanger, or restricted pipework can all make the return water remain too hot. It can also be caused by a failed temperature sensor, wiring fault, or a problem on the control PCB that is showing an incorrect reading. Severity is moderate to high: overheating can damage the heat exchanger and other parts, so the fault should not be ignored. While there are a few safe checks a competent homeowner can do, most diagnostic and repair actions (pump replacement, heat exchanger work, sensor or PCB replacement, work on gas/electrical parts) must be carried out by a Gas Safe registered engineer. If the fault clears after a single reset and normal operation resumes, monitor closely. If the E9 285 returns, or if you see other warning signs (burning smell, leaks, repeated trips), switch the boiler off and call a qualified engineer. Do not keep resetting and running the boiler with a persistent overheat fault.
Possible Cause: The boiler return temperature sensor has measured a temperature higher than 95°c.
Troubleshooting Steps
Safety precautions:
1. Before doing any checks, turn the boiler off using the front panel switch and allow it to cool down. Do not strip or open the boiler casing. Do not attempt any gas or electrical repairs yourself. If you smell gas, leave the property immediately and call the gas emergency service.
2. Only perform non-invasive visual checks and simple user-level operations such as reset, checking pressure, and listening for pump noise. For anything that requires opening the boiler, testing wiring, or replacing components, call a Gas Safe engineer.
Initial checks a homeowner can safely do:
1. Note the exact fault code and any secondary cause code displayed. Take a photo of the display for the engineer.
2. Check the boiler pressure on the pressure gauge. Normal cold pressure is typically around 1 to 1.5 bar. Very low pressure can affect operation; very high pressure can indicate other problems. Do not try to fix pressure issues if you are unsure; follow the manual or wait for the engineer.
3. Try a controlled reset: consult the manual for your GB162 reset method (some models have a reset button to press for about 3 seconds, others use a dial reset). Perform one reset and then watch whether the fault returns. Do not repeatedly reset a persistent overheat fault.
4. Look and listen: are radiators hot at the flow side and cooler on the return side? Is the pump running (you may hear a low whirr or feel a slight vibration at pump casing if accessible and safe)? Any loud banging or unusual noises? Any visible leaks or signs of burning?
Specific diagnostic steps and simple remedies you can try (non-invasive):
1. Turn the heating off and allow the system to cool. After cooling, turn the boiler back on and try a single reset as above. If the error does not return, monitor the system closely over the next hours/days.
2. Check thermostats, zone valves and programmer settings to ensure the boiler is not being forced to an abnormal condition (e.g. all zones closed while boiler tries to run). Ensure any external controls/cylinder thermostats are calling correctly.
3. Bleed radiators to remove trapped air from the system. Airlocks can sometimes reduce circulation and cause local overheating. After bleeding, check pressure and repressurise only if you are confident following the boiler manual instructions.
4. If you can safely access pipework near the boiler, carefully feel the flow and return pipes (when the system has been running and is warm). The flow should be significantly hotter than the return. If both are equally hot, circulation is poor (possible stuck/blocked pump, closed valve, or sludge/limescale issue).
5. If you suspect a frozen condensate or external pipework issue in cold weather, thaw gently with warm (not boiling) water. This is more commonly related to condensate blockages and will not usually produce E9 285, but it is a simple check in freezing conditions.
When to call a professional (and what they will check):
1. If the E9 285 persists after a single reset, or if the fault returns repeatedly, switch the boiler off and contact a Gas Safe registered engineer. Do not continue to run the boiler.
2. The engineer will check pump function and flow/return temperatures, inspect and test the return temperature sensor and its wiring, examine the safety temperature limiter, check for limescale or sludge in the heat exchanger, test flow rates, and assess the PCB for faults. Repairs often require component replacement (pump, sensor, safety limiter, heat exchanger or PCB) and specialist tools and parts.
3. Prepare useful information for the engineer: the model and serial number, the exact fault code(s) and any cause codes, when it started, any recent maintenance or changes to the heating system, and photos of the display.
Important warnings:
1. Do not attempt to remove or replace sensors, open the boiler, or work on gas/electrical parts yourself. This is unsafe and illegal in many jurisdictions unless you are a registered engineer.
2. Do not repeatedly reset the boiler to force it to run if the overheat fault keeps returning—this risks further damage.
3. If you detect a gas smell, CO alarm activation, visible overheating, or severe leakage, turn the appliance off, ventilate the property, evacuate if necessary, and call emergency services/gas emergency line and a Gas Safe engineer.
Summary: You can perform basic checks: note codes, try a single reset, check pressure, bleed radiators and check for pump noise and obvious circulation issues. Most likely causes for E9 285 (blocked/failed pump, poor circulation, heat exchanger scale/sludge, faulty sensor or PCB) require a Gas Safe registered heating engineer to diagnose and repair safely.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.