Overview
The E9 220 fault on a Worcester Bosch GB162 means the boiler’s safety temperature limiter / safety temperature sensor has gone into a protective lockout. The specific cause code 220 indicates either the sensor contacts have shorted (electrical short) or the sensor has reported an abnormally high flow temperature (an indicated temperature well above normal operating limits). In most Worcester boilers the safety limiter is designed to prevent central heating flow water overheating (commonly cited as tripping around 105°C), but the 220 cause points to a sensor/contacts issue where the control board is seeing an extreme/high reading (or a short) that forces the boiler to shut down. This is a high-severity safety fault. Overheat conditions and sensor short circuits can damage internal components and must not be ignored. It is not a safe DIY repair: you can carry out a small number of non-intrusive checks listed below, but any electrical or gas-side diagnosis or part replacement must be performed by a qualified, Gas Safe registered engineer. Repeatedly resetting the boiler without diagnosing the root cause is unsafe and should be avoided.
Possible Cause: The contacts for the safety temperature sensor have shorted or the safety temperature sensor has detected a temperature higher than 130°c.
Troubleshooting Steps
Safety precautions:
1. Do not attempt internal electrical or gas repairs. Only a Gas Safe registered engineer should work inside the boiler or on gas pipes.
2. If the boiler is displaying E9 220, switch the boiler to the OFF position and isolate the electrical supply at the fuse or switch if you can do so safely. Do not operate the boiler until the fault is investigated.
3. If you smell gas, evacuate the building immediately and call the gas emergency number. Do not attempt to reset or light the boiler.
4. Do not repeatedly press reset—one attempt to reset for test is acceptable, but repeated resets can mask a developing or dangerous fault.
Initial homeowner checks you can safely perform:
1. Note and record the full fault display exactly as shown (E9 and the 220 cause). Record any other behaviour (noise, leaks, smells, whether CH or hot water is affected).
2. Check system pressure on the boiler display. If pressure is very low (below ~0.8 bar) or very high, note this — pressure problems can affect circulation but are not the primary cause of E9 220.
3. Check radiators and heating demand: are some radiators cold while the boiler shows fault? This can indicate poor circulation or a seized/blocked pump.
4. Bleed one or two radiators (starting with the highest cold radiator) if you suspect trapped air; bleeding can restore circulation in some cases.
5. Look externally for signs of water ingress, corrosion or obvious damaged wiring to the boiler casing or sensors (do not open the boiler). If there is visible water around the case or electrical connectors, switch off power and note that for the engineer.
6. If accessible and safe, check for obvious blockages to the condensate pipe (normally a frozen or blocked condensate causes different faults but clearing an external frozen drain is harmless). Do not attempt to thaw inside the boiler or dismantle pipework.
Diagnostic steps and what an engineer will do (what you should expect them to check):
1. The engineer will interrogate the boiler memory and confirm the cause code and any history of faults. Provide the E9 220 record and any observations you made.
2. They will check the safety temperature limiter / sensor wiring and connector for signs of a short, loose terminals, water ingress or chafing. If contacts are shorted or corroded the sensor or connector will usually be replaced.
3. They will measure the actual temperature at the flow/CH sensor location with diagnostic tools to confirm if the sensor is giving a false high reading or if the boiler truly overheated.
4. Circulation checks: the engineer will verify the pump operation and flow through the heat exchanger. A blocked or seized pump will cause local overheating; the pump may be cleaned, unblocked or replaced.
5. Heat exchanger inspection: they will examine the heat exchanger for limescale or damage. Heavy limescale buildup reduces heat transfer and can cause overheating; a damaged exchanger may need replacement.
6. PCB and control checks: if sensor readings and wiring are correct yet the fault persists, the PCB and associated electronics will be tested. Faulty electronics can report incorrect temperature data or create spurious faults.
7. After repairs or component replacement, the engineer will carry out a safe restart, test the system under load and confirm no other faults are present.
Important notes and next steps:
1. Do not attempt to remove or replace the safety temperature limiter, sensors, pump or PCB yourself. These are gas appliance or mains-electric repairs and are legally required to be done by a qualified engineer.
2. If your boiler is under warranty, contact Worcester Bosch or your installer and have your engineer log the fault prior to any major work where possible—warranty may cover defective parts.
3. When you call for a repair, provide the engineer with the exact fault codes, boiler model (GB162), serial number, age of the boiler, and any recent changes to your heating system (new radiators, powerflushing, water top-ups, etc.).
4. If you have temporary loss of heating or hot water, plan for alternatives (extra layers, electric heaters, hot water via kettle) until a qualified engineer can attend.
Summary action for homeowner: turn the boiler off, make one safe reset attempt only if you wish to test, record the fault codes and observations, do the basic checks listed above (pressure, bleeding radiators, visible leaks), and call a Gas Safe registered heating engineer to diagnose and repair the E9 220 fault. Do not carry out internal boiler repairs or replacements yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.