Overview
The F1 263 fault on a Worcester Bosch Greenstar i (system/combi) indicates a communication or installation fault involving the Heat Control Module (HCM) or the boiler control box. In simple terms the boiler is telling you that the HCM is either not correctly seated/installed or is defective, or that the control box it talks to is faulty or out of sync. When the HCM and control box cannot communicate correctly the boiler can lock out or refuse to run until the fault is resolved. Severity is moderate to high: this is not a simple comfort fault and can prevent the boiler from operating normally. It is not a routine DIY repair because the HCM and control box are internal electronic components that require correct identification, handling and re‑installation, and work on gas appliances must be done by a Gas Safe registered engineer. Some basic checks and a reset can be done by a homeowner, but diagnosis and replacement of the HCM or control box should be left to a qualified engineer.
Possible Cause: Control box or the heat control module (HCM) is defective.
Troubleshooting Steps
Safety precautions:
1. If you smell gas, leave the property immediately and call the gas emergency number for your country. Do not operate electrical switches or the boiler.
2. Only perform non-invasive checks. Do not remove the boiler cover or attempt internal repairs unless you are a Gas Safe registered engineer.
3. Isolate the boiler electrically (switch off at the fused spur or mains) before touching any external wiring or moving the boiler. When in doubt, wait for a qualified engineer.
Initial homeowner checks you can do:
1. Note the exact fault display and any other codes or messages. Use the boiler's quick info or information menu if available to record the cause code.
2. Try a controlled reset: either press the reset button for 3 seconds or follow the model's reset procedure (turn the temperature dial to the reset position and hold for 3 seconds where applicable). After reset, allow a minute and see if the code returns.
3. Check that the boiler has power: confirm the fused spur is on and any external timers or room thermostats are calling for heat. If external controls are off, the boiler may not fire but should not show an F1 263 cause repeatedly.
4. Check whether the fault appeared immediately after any recent service or part replacement. If the fault started after work on the boiler (new HCM or control box fitted), this increases the likelihood of an incorrect or poorly seated module.
If the fault persists (recommended progression for an engineer):
1. Do not continue to attempt internal fixes if you are not qualified. Contact a Gas Safe registered engineer or Worcester Bosch support and provide the boiler model, serial number and the F1 263 cause code.
2. The engineer will isolate gas and electrical supplies, remove the boiler casing and inspect the HCM and control box connections. They will check the HCM is the correct part number for that boiler, correctly oriented and fully seated on its connector, and that no pins or sockets are damaged or corroded.
3. The engineer will inspect and test the control box connector, ribbon cables and related wiring for continuity, damage or poor contacts. Communication and synchronization between the HCM and control box will be tested, and any faulty connectors replaced or repaired.
4. If the HCM or control box is confirmed faulty, the engineer will replace the defective module with a correct, matching part and ensure firmware/part compatibility. After replacement they will clear fault logs, power the boiler up and verify normal operation and communication through a full system and ignition sequence.
5. The engineer should also check for any secondary issues that could have contributed (e.g., recent water ingress, overheating, physical damage, or incorrect retrofit parts).
Important notes and when to call a professional:
1. This fault involves internal electronic modules and possible gas isolation—do not attempt to open or repair these components yourself. A Gas Safe registered engineer must carry out internal diagnostics and replacements.
2. If a reset clears the code temporarily, book an engineer anyway: intermittent communication faults can reappear and may damage components or leave the system unreliable.
3. Provide the engineer with any relevant history (recent servicing, part changes, when the fault started) to speed diagnosis.
If you need immediate help, contact a Gas Safe registered heating engineer or Worcester Bosch customer service. Keep the boiler paperwork and serial number ready for the engineer to help identify correct replacement parts.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch Greenstar I System / Combi Boiler.