Error E40

Safety first:

- If you are not competent with gas or mains electricity, stop and call a Gas Safe registered engineer. Working inside a boiler can be dangerous and may void warranties.

- Before touching anything, turn the boiler off at the programmer/room thermostat and isolate electrical supply at the mains or the boiler fused spur. Allow the boiler to cool.

- Do not attempt to work on the gas valve, burner, or any live mains wiring unless you are qualified.

Initial checks a homeowner can do:

1. Note the exact behaviour and any other codes showing on the display. Record when the fault appears (on heat demand, on DHW, at start-up).

2. Try a basic reset: switch the boiler off, wait 30 seconds, and switch it back on. If E40 clears but returns, the fault is persistent.

3. Check system pressure on the gauge. If pressure is very low (<1.0 bar) repressurise to the recommended level (typically ~1.2–1.5 bar) following your boiler manual; very low pressure can cause circulation problems that indirectly affect sensors.

4. Observe whether radiators and hot water are heating normally. Poor circulation, cold radiators or noisy pump may indicate a separate circulation issue to mention to the engineer.

5. Visually inspect the external area around the boiler for obvious water leaks, corrosion or disconnected wiring. If you see water near wiring or connectors, isolate power and do not attempt to run the boiler.

If you are comfortable with basic electrical isolation and have a multimeter (only proceed if you understand safe isolation procedures):

1. Isolate mains and confirm the boiler has no power. Remove the boiler casing only if you are confident and the manual allows access; some panels may be secured for safety and warranty.

2. Locate the central heating return thermistor connector and visually inspect for loose pins, corrosion, or damaged cable sleeving. Reseat the connector if it is a plug-and-socket type.

3. With the power still isolated, check wiring continuity between the sensor connector and the PCB connector to rule out a broken conductor. An open circuit indicates wiring or connector failure.

4. Remove the sensor from its pocket or mounting (if accessible) and measure its resistance at ambient temperature with the multimeter. Thermistors are NTC type (resistance falls as temperature rises). The exact resistance curve varies by model; compare to the value in your service manual if available (many NTCs used in boilers are around 10 kΩ at 25 °C but confirm before relying on that). A very high reading (open circuit) or zero/near-zero (short) indicates a faulty sensor.

5. A simple functional check: if safe to do so, dip the sensor tip briefly into warm water and watch the resistance change—resistance should drop as the sensor is warmed. Do not immerse electrical connector parts and keep everything dry.

Possible fixes and next steps:

- If the connector is corroded or loose, cleaning and securely reseating it may clear the fault. Ensure power is isolated before doing this.

- If the sensor shows open circuit or does not change value when warmed, replacing the return thermistor is the usual remedy. Sensor replacement is a straightforward mechanical task for an engineer but may be more than a homeowner should attempt depending on access and skill.

- If wiring is broken between the sensor and PCB, wire repair or replacement is needed.

- If the sensor and wiring check out, the PCB input channel may be faulty and will need professional diagnosis and repair or board replacement.

- If there are circulation symptoms (cold radiators, pump noise, frequent overheat codes), address circulation first (bleed radiators, check pump operation) and report these findings to the engineer — circulation issues can cause false sensor readings or overtemperature conditions.

When to call a professional:

- Call a Gas Safe registered heating engineer if E40 persists after a reset and simple visual checks. Give them the boiler model and the E40 code and tell them what you have already checked (pressure, radiator behaviour, any visible damage).

- Only a qualified engineer should replace sensors, test and repair internal wiring, or work on the PCB or gas components. If the fault is intermittent, document when it happens to help the engineer diagnose.

What to tell the engineer when they arrive:

- Boiler model and serial if available, the exact fault code E40, what you observed (when it happens, whether it clears after reset), system pressure reading, radiator behaviour, and any visible signs (water, corrosion, disconnected plugs).

Important final note:

- Do not continue to use the boiler if you suspect internal water leaks, exposed wiring, or unsafe operation. A persistent sensor fault should be repaired promptly by a qualified engineer to ensure safe, efficient heating system performance.