Worcester Bosch Boiler Fault Codes: Causes & Fixes
Unusual letters and numbers on your Worcester Bosch boiler display aren’t random — every fault code points to something specific. Some you can fix yourself in minutes. Others need a Gas Safe engineer the same day. This guide covers every common Worcester Bosch fault code, what it means, and exactly what to do about it.
Important: If your boiler displays a fault code and you’re unsure, always err on the side of caution. Gas, electrical, and pressure faults should never be investigated by anyone who isn’t qualified.
How to Use This Guide
Find your fault code in the sections below. Each entry explains the likely cause and the appropriate response. Some codes can often be resolved without an engineer. Others should not be attempted yourself under any circumstances.
Worcester Bosch Fault Codes Explained
A-Series: Circulation and Pump Faults
A1 281 – Pump Stuck or Air in System
Circulation issue caused by low water pressure or an airlock in the system.
Fix: Repressurise your boiler to between 1 and 1.5 bar using the filling loop, then bleed your radiators to release trapped air.
B-Series: Control Board Faults
B7 257 – Internal Control Board Error
Likely a fault with the PCB (Printed Circuit Board) — the boiler’s main control unit.
Fix: A Gas Safe engineer will need to inspect and likely replace the PCB. Not a DIY repair under any circumstances.
C-Series: Fan and Airflow Faults
C1 264 – Fan Stopped During Operation
The fan has stopped working mid-cycle, which prevents safe flue gas expulsion.
Fix: Could be a blockage, wiring fault, or failed fan motor. Requires engineer inspection — do not attempt to restart repeatedly.
C6 215 – Fan Running Too Fast
Fan speed is exceeding safe operational limits, pointing to a sensor fault or airflow restriction.
Fix: Needs diagnosis by a Gas Safe engineer. May involve replacing the fan speed sensor or clearing an obstruction.
D-Series: Sensor Faults
D1 240 – Return Sensor Error
The return temperature sensor is wet, damaged, or faulty.
Fix: An engineer will test, dry, repair, or replace the sensor as needed.
E-Series: Temperature and Electrical Faults
E2 222 – Flow Sensor Short Circuit
An electrical fault in the flow temperature sensor.
Fix: Sensor replacement required — Gas Safe engineer only.
E5 218 – Flow Temperature Too High
The boiler water temperature is exceeding safe limits. Often caused by a pump issue, blocked radiators, or trapped air.
Fix: Check system pressure (should be 1–1.5 bar), bleed radiators, and ensure all radiator valves are open. If the fault persists, call an engineer to inspect the pump.
E9 219 – Safety Sensor Fault
Excessive temperature detected or safety sensor malfunction.
Fix: Professional sensor testing and full boiler inspection required.
E9 220 – Safety Sensor Short Circuit
The main heat exchanger temperature sensor has failed.
Fix: Sensor replacement by a qualified engineer.
E9 221 – Safety Sensor Open Circuit
Sensor not being recognised — likely a wiring fault or sensor failure.
Fix: Requires engineer diagnosis. Could be a loose connection or full sensor replacement.
E9 224 – Max Temperature Thermostat Activated
The flue gas thermostat has overheated, often due to restricted airflow.
Fix: Engineer should inspect the flue, clean the air filter, and check fan operation. May require thermostat replacement.
EA-Series: Ignition and Flame Faults
EA 227 – Frozen Condensate Pipe
One of the most common winter fault codes. The condensate pipe has frozen, causing the boiler to shut down safely.
Fix: Pour warm — not boiling — water over the external condensate pipe to thaw it, then reset the boiler.
EA 229 – Loss of Flame During Operation
The flame cut out unexpectedly, often due to a blocked condensate trap or moisture ingress.
Fix: An engineer should clean the condensate trap and inspect for water ingress into the combustion chamber.
EA 234 – Gas Valve Error
The gas valve coil has become disconnected or failed.
Fix: Engineer will reconnect or replace the gas valve. Do not attempt any gas component work yourself — this is a legal requirement.
EA 261 – Ignition Timing Error
No flame was established during the ignition period.
Fix: May involve adjusting ignition settings, cleaning the ignition electrodes, or replacing the igniter.
EA 338 – Too Many Short Burner Starts
The pump may not be detecting water — often caused by low pressure.
Fix: Check the pressure gauge and repressurise to between 1 and 1.5 bar. If the fault returns, call an engineer.
EF-Series: System Faults
EF 349 – Central Heating Boil Detected
The temperature difference between flow and return pipes exceeds 18°C — a sign of poor circulation.
Fix: Likely caused by a pump fault, sludge buildup, or system blockage. Requires engineer investigation.
9-Series: Module and Configuration Faults
9A 362 – Incorrect HCM Fitted
The Heat Control Module is not matched to your boiler model.
Fix: An engineer will replace the HCM with the correct compatible unit.
9U 233 – Heat Control Module Error
A code plug issue within the HCM.
Fix: Requires reprogramming or replacement of the control module by a qualified technician.
No-Code Faults
Code 212 – Safety or Flow Temperature Rising Too Fast
No fault code displayed but the boiler locks out due to an airlock or insufficient water.
Fix: Check the pressure gauge, repressurise if below 1 bar, and bleed radiators. If the issue returns, call an engineer.
When to Call a Gas Safe Engineer
Some fault codes — like EA 227 (frozen condensate) or A1 281 (low pressure) — are safe to resolve yourself. But codes involving gas valves, ignition, sensors, or the PCB are not DIY territory. Attempting to fix them without the right qualifications is both dangerous and illegal in the UK.
Never repeatedly reset a boiler that keeps returning the same fault code without identifying the underlying cause.
Worcester Bosch Fault Code Quick Reference
Dealing with a Worcester Bosch fault code right now? Call Plus Heat on 0808 164 2892 — our Gas Safe engineers are on hand to diagnose and fix boiler faults, and with a Plus Heat cover plan, breakdowns are covered before they catch you off guard.
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