Overview
D1 240 on a Worcester Bosch GB162 (Greenstar) means the boiler’s central heating return temperature sensor is reporting a short circuit. The control electronics are detecting that the sensor wiring or sensor itself is electrically shorted (for example the two sensor leads touching, a short to earth or a failed NTC sensor). The fault message can also be triggered if the sensor has been damaged, become wet from a leak or condensate, or if a connector or wiring loom has failed. Severity is moderate: this is a safety/protection fault that commonly causes the boiler to lock out or stop calling for heat or hot water until the fault is resolved. It is not usually an immediate gas explosion risk, but the boiler will not operate correctly and attempting internal electrical repairs without the right qualifications is unsafe. Some basic non-intrusive checks are suitable for a homeowner, but diagnosing and replacing sensors or wiring should be carried out by a qualified, Gas Safe registered heating engineer.
Possible Cause: The contacts of the boiler return temperature sensor have been short circuited.
Troubleshooting Steps
Safety precautions:
- If you are unsure or uncomfortable at any stage, stop and call a Gas Safe engineer. Do not attempt to work on gas or live electrical parts.
- Before any physical checks you can safely do, switch the boiler to the OFF position and isolate the electrical supply at the fused spur or consumer unit if you need to touch wiring or connectors. If you do not know how to isolate safely, do not proceed.
- Never remove sealed covers, the burner access, or internal panels on a gas appliance unless you are a qualified engineer.
Initial checks a homeowner can do (safe, non-intrusive):
1) Record the exact error code and any cause code shown in the information menu (press the spanner/info button if your boiler shows it) and note when it occurs (on heating, on hot water, continuously).
2) Try a simple reset: press and hold the reset button for the required time (usually 3 seconds) or follow the boiler manual reset procedure. If the fault clears and stays cleared, monitor for recurrence. If it comes back immediately or soon after, further action is needed.
3) Visually inspect around the outside of the boiler for signs of water leaks, damp, or condensation running from pipework, joints or the condensate trap. A wet sensor or water ingress can cause a short.
4) Check any visible external wiring or plug-in connections to programmers, room stats or external controls for obvious damage or loose plugs (only if these connectors are externally accessible and you do not need to open the boiler case). Do not pull apart or probe live plugs without isolating first.
Further diagnostic steps (if you are competent, comfortable and have isolated power; otherwise skip and call an engineer):
1) With power isolated, if there is an externally accessible sensor connector or terminal that does not require removing the boiler case, you can check that the connector is pushed home and free of corrosion or moisture. Dry and reseat the plug and restore power to test. If the connector is inside the boiler casing, do not open it — call an engineer.
2) If the boiler shows the fault only after heavy rain or after a period of condensate or leak evidence, drying out the area (once isolated) and reseating external connectors may give a temporary cure, but replacement of the sensor/connector will usually be required.
3) Do not attempt to measure sensor resistance or continuity on live circuits. These checks and any replacement sensors should be performed by a Gas Safe engineer with the correct test equipment and procedures.
When to call a professional and what to tell them:
- Call a Gas Safe registered heating engineer if the reset does not fix the fault, if the fault returns, if you find signs of water ingress, or if the connector or wiring is inside the boiler case.
- Give the engineer the boiler model (Worcester Bosch GB162 / Greenstar), the fault code D1 240 and any cause code, note when it happens (hot water, CH, or both), and any visual signs (leaks, corrosion, damp). Taking photos of the display and any visible wiring or leaks is helpful.
Why a professional is needed:
- The likely fixes include testing and/or replacing the return NTC sensor, repairing or replacing damaged wiring or connectors, and in some cases checking the control board. These tasks involve removing covers, working on live electrical circuits and gas components and must be done by a qualified engineer to be safe and to comply with regulations.
Do not attempt: replacing sensors inside the boiler, probing live circuits, or any work that requires opening the boiler casing unless you are a Gas Safe registered engineer. These actions are hazardous and can make the appliance unsafe or invalidate warranties.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.