Error E.00 – .04

Safety first

1) If you smell gas, leave the property immediately and call the gas emergency number. Do not attempt any checks.

2) Do not open the boiler casing or attempt internal electrical or gas repairs unless you are a qualified Gas Safe engineer. Internal work is a gas appliance safety issue.

3) Before touching any external controls, switch the boiler off at the programmer and at the isolator (or main fuse) if you can do so safely. Keep children and pets away while troubleshooting.

Initial checks a homeowner can do

1) Note the exact error code and any other lights or messages on the display. Make a clear note of when it occurred and any recent events (power cut, plumbing work, condensation freezing, servicing).

2) Try a controlled reset: press and hold the boiler reset button (or follow the manual reset procedure) for the recommended time (usually 3–5 seconds). Wait and see if the code returns. If it clears and the boiler runs normally for a while, monitor for recurrence.

3) Check boiler water pressure on the gauge—if pressure is very low (<0.5 bar) the boiler may behave oddly; if low re-pressurise per the manual, then recheck. Low pressure is not the same as a sensor open-circuit but is worth ruling out.

4) Look for obvious external signs: visible leaks, water on top of or around the boiler, obvious damage to wiring outside the casing, or evidence of condensation/icing around external pipes.

5) If you or a previous engineer recently removed the casing, check that any external connections you can safely access (room stats, external controls) are wired and operating normally. Do not open the boiler front panel.

Diagnostic and repair steps for a qualified engineer (overview)

1) Isolate gas and electrical supplies and follow safe procedures for working on the appliance.

2) Remove the boiler casing to access the return temperature sensor, its wiring and the PCB connector.

3) Visually inspect the sensor, connector and wiring for corrosion, water damage, chafing or a disconnected plug. Re-seat any loose connectors and inspect the PCB header for damage.

4) Measure the sensor resistance and compare to the manufacturer specification (service manual). Also check continuity of the wiring from the sensor back to the PCB to confirm whether the fault is in the sensor or the cable/connector. An open-circuit (infinite resistance) confirms a break/failed sensor.

5) If wiring is faulty, repair or replace the damaged cable and connectors. If the sensor is out of specification or open/shorted, replace the return NTC sensor and its sealing O‑ring. Ensure correct routing and secure connections to prevent repeat failure from vibration or moisture.

6) Check related systems that can produce misleading sensor readings: ensure the pump is circulating, no airlocks, and that flow/return connections are correct. A lack of circulation or incorrect plumbing can give temperature differentials that trigger faults.

7) After repair, restore supplies, clear error codes, and run the boiler through a heating and hot‑water demand to verify correct temperature readings and stable operation. Monitor for reappearance of the fault.

When to call a professional

If the fault does not clear after a basic reset, if you find any damage to internal wiring or components, or at any point you are uncomfortable, stop and call a Gas Safe registered engineer. Replacement of internal sensors, repairs to wiring or work inside the boiler must be done by a qualified engineer. Persistent or intermittent sensor faults often require replacement parts or further examination of the PCB and wiring by a professional.