Error E43

Safety precautions:

1) Always isolate the electrical supply to the boiler at the mains before removing the casing or touching internal components. Do not work on the boiler with power on. Turn off the gas supply at the external isolator if you will be removing covers or working inside the appliance. Allow the boiler to cool. If you are not confident or not qualified, stop and call a registered engineer.

2) Never operate the boiler with safety panels removed and do not reset the boiler repeatedly — one reset to test is acceptable, repeated resets can mask or worsen faults.

Initial homeowner checks (do these first):

1) Note when the fault occurred and whether any recent work or power surges occurred. Press the reset button once and see if the boiler clears and runs; if E43 returns immediately or shortly after, proceed to the checks below.

2) Check the boiler display wiring and any visible connectors on the outside of the casing for loose plugs, corrosion or moisture. Look for obvious signs of water leaks, rodent damage or chafed wiring near pipework.

3) If the boiler is installed with easily accessible pipework covers where the return sensor is clipped, check that the sensor is still clipped to the correct pipe and hasn’t been displaced.

Diagnostic steps for a competent DIYer with a multimeter (ONLY with mains power isolated):

1) Isolate mains power to the boiler and confirm it is dead with a multimeter or test lamp.

2) Remove the outer casing following the manufacturer’s instructions to gain access to the internals. Keep panels and screws safe.

3) Locate the return thermistor/sensor — it is usually clipped onto the return water pipe and has a 2- or 3-pin connector that goes to the wiring harness.

4) Visually inspect the sensor, plug and wiring for corrosion, damp, broken insulation, or crushed/chafed cables. If connectors are wet or corroded, drying and cleaning may help but replacement is usually advised.

5) With the sensor still connected or after unplugging it (follow wiring mark or take a photo so you can reconnect correctly), measure resistance across the sensor wires with a multimeter set to ohms. A short-circuited sensor will show a very low resistance close to 0 ohms (near short), whereas a healthy thermistor will show a finite resistance (not zero) and will change if you warm or cool the sensor slightly. Do NOT measure resistance with power on.

6) Also measure resistance between each sensor lead and earth/chassis. A low reading to earth suggests a leak/short to ground which will cause E43.

7) If the sensor measures as a short or shows continuity to earth, the sensor is faulty and should be replaced. If the sensor reads sensibly (not shorted) at the sensor connector, check continuity of the wiring back to the control board connector to ensure no short further along the harness.

8) If wiring to the board is shorted, inspect the wiring loom for damage and repair or replace the damaged section. If wiring appears intact but the control board connector shows a short at the board, the fault may be on the control PCB.

Fix steps and next actions:

1) If you identify the sensor as shorted, replace with the correct OEM replacement return sensor/thermistor. Fit the new sensor in the same position and orientation, connect the plug firmly, re-fit casing, restore power and test.

2) If connectors are corroded or wet, isolate power, dry and replace the connector or wiring as needed. Temporary drying alone is not a reliable long-term fix; replacement of damaged parts is recommended.

3) If sensor and wiring are good but the control board shows a short or the fault persists after replacing the sensor, the control PCB or its wiring harness may be faulty and will likely require a qualified engineer to diagnose and replace.

When to call a professional:

1) Call a Gas Safe (or your country’s equivalent) qualified heating engineer if you are not fully confident working safely inside the boiler, if the fault persists after basic checks, if you need sensor replacement, wiring repairs, or control board replacement, or if you suspect gas or other safety-related issues.

2) A professional will safely isolate, diagnose with manufacturer procedures, fit correct replacement parts (sensor or control box), re-commission the boiler, check combustion and run safety checks.

Final notes:

Do not bypass safety controls or continue using the boiler when it reports E43. The fault indicates an invalid temperature input and the boiler is protecting itself. Keep records of any troubleshooting and the error behavior for the engineer to speed diagnosis.