Error E40

Safety precautions:

1) If you smell gas, evacuate the property immediately and call the gas emergency number—do not attempt any checks. 2) Before touching the boiler, switch the boiler off at the control panel and isolate the electrical supply at the fused spur or consumer unit. 3) Do not attempt to work on the gas valve, burner, or sealed parts of the boiler unless you are a Gas Safe registered engineer. 4) If you are not confident with basic electrical checks, stop and call a Gas Safe engineer.

Initial homeowner checks (safe and simple):

1) Note exactly when the E40 appears (on power-up, on demand for heat, continuously). 2) Try a boiler reset: if the boiler has a reset button hold it until the display clears; if it uses an ON/OFF/SELECTOR switch, put it to R for about 5 seconds; for thermostat knob types turn fully anticlockwise then back. Record whether the code returns immediately or after calling for heating. 3) Check boiler pressure on the gauge—very low pressure can cause other faults; top up only if you know how and the pressure is low. 4) Look for obvious signs of water leaks, corrosion, or physical damage on visible pipework and sensor leads outside the boiler.

Visual and basic diagnostic checks (requires removing outer casing only if you are competent; isolate mains first):

1) With power isolated and the front panel removed, visually inspect the return NTC sensor location and wiring for loose connectors, broken wires, pinched cables, signs of water ingress, or corrosion. The return sensor is usually clipped to the return pipe or manifold inside the boiler. 2) Check the connector at the sensor and at the PCB terminal for tightness—loose terminal screws can cause intermittent faults. 3) Check for rodent damage or chafed wiring along the cable route.

Electrical checks only if you are competent and comfortable with basic multimeter use (isolate mains first):

1) Locate the sensor plug or disconnect point and measure continuity of the cable from the sensor connector to the PCB to confirm there is no open circuit. 2) With the sensor disconnected from the PCB and wiring isolated, measure the sensor resistance with a multimeter. You are looking mainly for an open-circuit (infinite resistance) or a short (near zero): either indicates a failed sensor. If the sensor shows reasonable variable resistance with temperature (you can warm it slightly by hand to see a small change), it is likely OK. 3) If wiring continuity is fine and the sensor resistance looks reasonable but the boiler still faults, the issue may be the PCB input or internal electronics.

Simple remedial steps you can try:

1) Re-seat any loose connectors found and re-tighten terminal screws. 2) If a sensor lead is damaged and you are competent to replace a short length of cable, ensure you use the correct type of cable and insulation and re-check continuity; otherwise do not attempt and call an engineer. 3) After any change, replace covers, restore power and attempt a normal restart and call for heating to see if E40 clears.

When to call a professional and what to tell them:

1) Call a Gas Safe registered engineer if the code returns after reset, if you find an open/short sensor, if wiring or terminals are corroded, or if you are not comfortable performing the checks above. 2) Do not attempt to remove or replace sensors that are inside the hydraulic/gas sections of the boiler or to access the burner or gas valve. 3) When you contact an engineer, tell them: the boiler model (Baxi 600 Combi), the error code E40, when it appears, what you tried (reset, visual checks, any continuity or resistance readings if taken), any signs of leaks or damage, and whether other fault codes are present.

Final note: E40 commonly ends up requiring replacement of the return NTC sensor or repair of its wiring, and sometimes inspection or repair of the PCB input. These are tasks for a Gas Safe registered engineer to carry out safely and to ensure correct commissioning and testing after repair.