Error 40

Safety precautions:

1) If you smell gas, leave the property immediately and call the gas emergency number. Do not attempt any checks. 2) Turn off the boiler at the programmer/controls and switch off the electrical supply at the isolator if you will be opening covers. Only open covers if you are confident and the boiler is isolated; otherwise leave this to an engineer. 3) Take care around hot pipework and sharp edges. If you are unsure at any point, stop and call a Gas Safe engineer.

Initial checks a homeowner can do:

1) Note the exact code and any other symbols on the display and whether the boiler is locked out. 2) Try a simple reset: switch the boiler off using the main switch, wait 30 seconds, then switch it back on or use the reset button following the manual. If the fault returns immediately or within a short time, further investigation is needed. 3) Check system pressure on the boiler gauge; very low or very high pressure can cause abnormal sensor behavior. If pressure is outside the normal range, follow your boiler manual to repressurise only if you are confident doing so.

4) Visually inspect the area around the boiler for signs of water leaks, corrosion, or cable damage. Note any recent plumbing or servicing that may have disturbed the sensor.

Specific diagnostic and fix steps (homeowner-level and what an engineer will do):

1) Locate the return sensor: it is usually fitted to the return pipework or mounted inside the boiler close to the heat exchanger. If the sensor or its connector is externally accessible and the boiler is isolated, you can look for a loose or disconnected plug. Do not pull on wires or force components. 2) If you see a visibly disconnected or corroded connector and you are competent with basic tasks, carefully reseat the connector and ensure it is dry and secure. Power the boiler back up and check if the error clears. If unsure, stop and call an engineer.

3) If the code remains, avoid further internal checks and contact a Gas Safe registered engineer. The engineer will: a) isolate and open the boiler safely; b) measure the probe resistance and compare to manufacturer specifications with a multimeter to confirm probe failure or wiring fault; c) inspect and continuity-test wiring between the probe and PCB, and check connectors for corrosion or water ingress; d) replace the return sensor if it is faulty, or repair/replace damaged wiring or connectors; e) check related sensors (flow sensor) and the control PCB for faults if the problem is not the probe; f) run the boiler through a full diagnostic and commissioning procedure and clear the error code.

When to call a professional and what to tell them:

1) Call a Gas Safe registered engineer if the fault persists after a reset, if you find damage, if you are not comfortable opening the boiler, or when repairs are required. 2) When you call, tell them the boiler make and model, the exact error code 40, what you saw on the display, whether you tried a reset, the system pressure reading, and any recent work or changes to the heating system.

Important notes:

1) Do not attempt repairs involving gas valves, burner components, or internal electrical wiring yourself. 2) No cost estimates are provided here; the engineer will advise required repairs and replacement parts. 3) If the boiler has locked out and you need heating or hot water urgently, an engineer can advise on safe temporary options or priorities for repair.