Error D.4

Safety precautions:

1. If you smell gas, leave the building immediately and contact your gas emergency service. Do not operate electrical switches or the boiler.

2. Before any visual inspection switch the boiler off using its power switch and isolate the electrical supply at the fused spur. Do not remove internal covers or components unless you are a Gas Safe engineer.

3. Never attempt to repair the gas or sealed parts of the boiler yourself. Internal troubleshooting and replacement of sensors or wiring must be done by a Gas Safe-registered engineer.

Initial homeowner checks (safe to do):

1. Try a simple reset: switch the boiler off, wait 30 seconds, and switch it back on. Check if the D.4 clears. If it returns, do not continue trying multiple resets.

2. Check the boiler display for any accompanying error codes (e.g. F.91) or messages and note them for the engineer.

3. Ensure central heating and hot water demands are actually active: check room thermostat settings, programmer/timers, and any external thermostats or controls that could be telling the boiler not to fire.

4. Check visible pipework and the condensate discharge for obvious damage or blockages and confirm system pressure on the boiler gauge is within the normal operating range (typically around 1–1.5 bar when cold). Low pressure can cause other faults; top up only via the filling loop following the boiler manufacturer instructions.

5. Visually inspect any accessible external wiring and connectors to the boiler (e.g. thermostat wiring, motorised valve connectors) for loose or damaged external cables. Do not open the boiler casing or touch internal connectors.

What a Gas Safe engineer will do (diagnostic and fix steps they will follow):

1. Isolate the appliance and open the boiler casing safely. Visually inspect inside for loose connectors, water ingress, burn marks or damaged harnesses.

2. Check the plug connections at the flow (warm start) NTC and at the PCB / actoSTOR electronics for poor seating or corrosion and reseat them securely. Inspect wiring harness for chafing or cuts.

3. Measure the resistance of the flow/warm-stat NTC at room temperature with a multimeter and compare to the expected NTC resistance chart for that sensor. This identifies open circuits, short circuits or drifted sensors.

4. Check continuity of the sensor cable from sensor connector to PCB to find any open or shorted sections and test for shorts to earth.

5. If applicable, test other related sensors (return, cylinder) and compare readings to ensure sensors are not swapped or incorrectly fitted.

6. Examine the PCB input for the sensor and test the board’s sensor-reading circuit. Replace or repair the PCB if it is confirmed faulty after verifying wiring and sensors.

7. Replace the sensor or the entire wiring harness if defective. Re-test the circuit and confirm the boiler reads plausible temperatures and runs through normal firing sequences.

8. If the fault is related to flue gas temperature limiter, STB or soot or other safety devices, the engineer will inspect, clean and replace the limit if necessary and check flue integrity.

9. Run the boiler under demand and observe operation, verifying the error does not recur and checking for correct pump, valve and burner behaviour.

Final notes and when to call a professional:

1. If the D.4 error persists after the basic homeowner checks, switch the boiler off and call a Gas Safe-registered heating engineer. This fault involves sensors, wiring and sometimes PCB components which require specialist testing and safe handling.

2. Avoid prolonged attempts to run the boiler with sensor faults; the boiler may repeatedly lockout or operate unsafely.

3. Provide the engineer with the exact error code(s), any recent work or changes (e.g. electrical work, thermostat replacement) and the steps you have already tried — this speeds diagnosis and repair.