Error E.04 – .01

Safety precautions:

1) Do not attempt gas work or replace gas-carrying parts unless you are a Gas Safe registered engineer. Working on gas appliances without registration is illegal and dangerous.

2) Before opening the boiler, switch the boiler off at the mains isolate and allow the boiler to cool. Isolating the electrical supply prevents electric shock and accidental boiler operation while exposed.

3) If you smell gas at any time, leave the property immediately and call the gas emergency number. Do not operate electrical switches or the boiler.

Initial homeowner checks (safe, non-invasive):

1) Note the exact error code and when it appeared. Try a simple reset: press and hold the boiler reset button as per the manual (usually 3–5 seconds). If the fault clears and does not return, monitor the boiler — if it reappears, further investigation is needed.

2) Check system pressure on the pressure gauge. Ensure pressure is within the normal operating range (typically about 1–1.5 bar when cold). Low pressure can cause circulation faults and related sensor errors.

3) Check if the heating pump appears to be running (you may hear it or feel gentle vibration). Make sure thermostatic valves and zone valves are open and that radiators are not fully air‑locked. Poor circulation can cause temperature read errors.

4) Look for signs of recent power surges, water leaks or condensation around the boiler which may have affected wiring or sensors.

Safe diagnostic steps a competent homeowner might perform (only if comfortable with basic electrics and isolating power):

1) Isolate mains power to the boiler before removing the outer casing. Wait for panels to cool and for stored energy to dissipate.

2) With the casing removed, visually inspect the flow temperature sensor wiring and connector (the sensor is usually mounted in the flow pipe or close to the heat exchanger). Look for loose connectors, damaged insulation, corrosion, water ingress or burnt wiring. Do not pull hard on wires.

3) If you find a loose connector, you may gently reseat it ensuring a firm connection, then replace the casing, restore power and test. If the fault clears, monitor the boiler. If the fault returns, do not continue to run the boiler — call an engineer.

4) If wiring looks damaged or wet, or any connector appears corroded or burnt, switch the boiler off and call a Gas Safe engineer. Do not attempt to repair wiring on a gas appliance unless qualified.

Diagnostics and fixes that should be performed by a qualified engineer:

1) Using appropriate meters, the engineer will measure the resistance of the flow NTC sensor and compare it to expected values and temperature behaviour (resistance should change with temperature). A short-circuited sensor will show abnormally low resistance or a direct short to earth or to another conductor.

2) The engineer will check continuity of the wiring loom, test for short circuits between sensor wires and to earth, and inspect/replace any damaged connector blocks.

3) If the sensor is faulty, the engineer will replace the flow temperature sensor and retest the system. If wiring or connector faults are found, they will be repaired or replaced.

4) If the sensor and wiring are good, the engineer may test the boiler PCB inputs to confirm the control board is reading correctly; a PCB fault can also present as a sensor short and will require professional repair or board replacement.

5) The engineer will check system circulation, pump operation and hydraulic conditions to ensure the fault was not indirectly caused by no/insufficient circulation.

When to call a professional:

1) Call a Gas Safe registered engineer if the fault does not clear after a reset, if you find any damaged wiring, corrosion, water ingress, or if you are unsure at any stage. Replacing sensors, repairing wiring, or working on the PCB must be done by a qualified technician.

2) If the boiler locks out repeatedly or intermittently, do not continue to run it; get a professional inspection to prevent overheating, component damage or unsafe operation.

Final notes:

- Document when the fault occurred and any actions you took (resets, observations) — this helps the engineer diagnose recurring problems.

- Avoid attempting invasive repairs on gas appliance internals if you are not trained; the safest and correct resolution for E.04 – .01 is usually professional inspection and replacement of the faulty sensor or repair of the wiring/connector.