Overview
The D1 241 error on a Worcester Bosch GB162 indicates an interruption or open circuit in the boiler return temperature sensor circuit — in plain terms the boiler is not getting a valid signal from the return-water NTC sensor (either the sensor, its wiring or connector is disconnected, damaged or wet). The boiler uses that sensor to monitor return-water temperature and to control burner modulation and safety limits. If the control cannot read the return temperature it will often block or lock out operation (or delay running) to protect the appliance and the system. Severity is moderate: this fault will typically prevent normal heating or the boiler operating reliably, but it is not an immediate life-safety emergency like a gas leak or CO alarm. However, because it affects control and safety functions you should treat it as a fault that needs prompt attention. Some very simple checks can be done by a competent homeowner (visual checks, reset), but diagnosing and repairing wiring or replacing sensors inside the boiler should be done by a qualified Gas Safe engineer. Working inside a gas-fired appliance has risks and may invalidate warranties if not carried out by an authorised technician.
Possible Cause: The contacts of the boiler return temperature sensor have been interrupted.
Troubleshooting Steps
Safety precautions:
1. If you suspect a gas leak, smell gas, or see signs of combustion problems, leave the property and call the gas emergency number immediately. Do not attempt repairs.
2. Before any inspection on the boiler, switch the boiler off at the external isolator and turn off the electrical supply at the fusebox. If you are going to isolate gas for any reason, do not relight the appliance yourself unless you are qualified.
3. Do not remove sealed burner or gas-carrying components. If you are uncomfortable with any step, stop and call a Gas Safe registered engineer.
Initial homeowner checks (safe, non-invasive):
1. Make a note of the exact fault code (D1 241) and any other codes or symbols shown on the display. Record when the fault started and any events (power cuts, freezing weather, recent work on the system).
2. Try a basic reset: follow the boiler manual reset procedure (press reset/spanner for 3 seconds or the model-specific reset). Wait a few minutes to see if the fault clears. If it returns, proceed with checks below.
3. Check the boiler pressure gauge and system pressure. Low system pressure can cause unrelated faults; top up to the recommended pressure if low and then re-check for the fault.
4. Look for visible signs of water ingress, corrosion or damp on the external wiring and around the boiler. A wet sensor or connector (from leaks or condensation) can cause an open/short.
5. Visually check accessible connectors: on some installations there is an external sensor lead or a terminal block that is accessible without removing sealed panels. If you can safely access a plug-in connector for the return sensor, ensure it is seated correctly and not obviously damaged. Do not force or open panels marked as service-only.
Specific diagnostic steps (for competent DIYers only; electrical isolation required):
1. Isolate electrical supply to the boiler at the mains and confirm it is dead before touching internal wiring.
2. With the boiler isolated, open any service cover only if you are confident and the manual permits. Inspect the sensor wiring that runs from the return pipe to the PCB: look for broken insulation, chafing, kinks, or a disconnected terminal.
3. Using a multimeter set to measure resistance/continuity, check the sensor circuit:
- Measure continuity between the sensor connector and the control PCB connector to check for a broken core in the cable. An open circuit indicates a broken wire or disconnected plug.
- Measure the resistance across the sensor itself (at room temperature). Many Worcester NTC sensors show a few kilo-ohms at ambient temperature (check the manual for the exact expected value for your model). An infinite resistance (OL) means open circuit; a very low resistance near zero may indicate a short.
4. If the wiring and connectors are intact but the sensor resistance is out of expected range, the sensor is likely faulty and needs replacement.
5. If a connector was wet or corroded, drying and cleaning the connector may temporarily clear the fault, but water damage usually requires component replacement and investigation of the water source.
Fix steps and when to call a professional:
1. If a simple re-seating of an external connector or clearing visible corrosion fixes the code and it does not return, monitor the boiler closely and contact an engineer if it reappears.
2. If the sensor is confirmed open/shorted, if wiring between sensor and PCB is damaged, or if you are required to remove internal covers to access components, stop and arrange for a Gas Safe engineer to attend. Replacement of NTC sensors, internal wiring repairs, and any adjustments involving gas or sealed parts must be performed by a qualified technician.
3. When you call the engineer, tell them the fault code (D1 241), what checks you have already done (reset, pressure check, visual inspection, any multimeter readings), and whether the connector was damp or visibly damaged. This speeds diagnosis.
4. After repair, the engineer will reset the fault, verify correct operation across heating and hot water modes, and check for any underlying causes (leaks, cable routing issues or control board faults).
Final notes:
- Do not repeatedly reset the boiler as a long-term workaround; that can mask a fault and may stress components.
- Because this error affects temperature sensing and safety control, the safest course for any internal repair or component replacement is to use a Gas Safe registered engineer. Provide the engineer with the error code and the observations from the steps above to help them diagnose and rectify the issue efficiently.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.