Error E40

Safety precautions:

1. If you are not competent working with gas appliances or electrical equipment, do not open the boiler or attempt internal repairs. Call a Gas Safe registered engineer.

2. Before any visible checks, put the boiler into its OFF position at the control panel and switch off the electricity at the isolator. Do not work on the boiler while live unless you are a trained engineer.

3. Never attempt to modify gas or sealed components on the boiler yourself.

Initial homeowner checks you can safely do:

1. Note the exact fault code, the conditions when it appears (e.g. when calling for heat), and any recent events (power cut, servicing, freezing conditions). This information helps the engineer.

2. Check boiler pressure. If the system pressure is very low (below about 1.0 bar) top up per your boiler manual; low pressure can cause circulation issues that complicate diagnostics. If you’re unsure how to repressurise the system, do not force anything—call an engineer.

3. Try a boiler reset using the R (Reset) position or reset button on your boiler control. Hold the reset for at least 5 seconds. If the E40 clears and stays cleared and heating works normally, monitor the system. If the fault returns, do not repeatedly reset — call an engineer.

4. Visually inspect for obvious external damage: look at any visible pipework and the area around the boiler for signs of leaks, corrosion or physical damage. Do not remove the boiler casing or access internal components.

Diagnostic and fix steps a professional will take (for your information):

1. Verify the fault history and replicate the fault conditions. The engineer will use the boiler’s fault log and run the boiler to observe behaviour.

2. Isolate power and access the return thermistor and its wiring. The engineer will check for loose connectors, damaged cable, corrosion or broken wires and repair or reterminate connections as required.

3. Measure the NTC sensor resistance with a multimeter and compare readings to Baxi manufacturer specifications or temperature-resistance charts. If the sensor resistance is out of range or intermittent, the thermistor will be replaced with the correct OEM part.

4. Check continuity and wiring back to the PCB to rule out wiring harness or circuit board connection faults. If wiring is fine but the PCB input does not read correctly, further PCB diagnostics are required.

5. After any repair or sensor replacement the engineer will reassemble the boiler, restore power and gas, and run the boiler through a full operating cycle to ensure correct temperatures, proper circulation and that the E40 fault no longer appears.

6. If the fault is due to circulation problems or unrelated system faults (airlocks, pump failure, very low pressure) these will be diagnosed and addressed as part of the visit.

Important final notes:

1. If E40 persists after a reset, or if you are not confident performing the safe checks above, contact a Gas Safe registered engineer. Replacing sensors, working on internal wiring or adjusting PCB settings must be done by a trained professional.

2. Record the engineer’s findings and any parts fitted. If your boiler is under warranty or a service plan, advise the provider of the fault code and actions taken.

3. Do not attempt to bypass safety devices or reset repeatedly to force the boiler to run; this can mask underlying faults and lead to further damage.

Call a Gas Safe engineer whenever the fault cannot be resolved by the safe homeowner checks above. They will diagnose, replace the thermistor or wiring if needed, and verify the boiler is safe to operate.