Error D.9

Safety first: do not open or work inside the boiler unless you are qualified. Turn off the boiler at the front panel and isolate mains power before touching any wiring. If you suspect a gas leak or smell gas, turn off the gas supply, leave the building and call the emergency gas number. Any internal electrical, gas or PCB work must be done by a Gas Safe registered engineer.

Initial homeowner checks you can safely do:

1) Note any other fault codes shown and the exact conditions when D.9 appears (e.g. on initial start, when calling for heat, after a reset). This information will help the engineer. 2) Try a simple reset: switch the boiler off at the front, wait 30 seconds, switch it back on and see if the code returns. 3) Check the external controller/thermostat: if you have an analogue regulator or third-party controller wired into the analogue input terminals, verify its settings and that it is powered and connected correctly. Temporarily set the external controller to a neutral position or disconnect it at the external controller end (not at internal PCB unless you are competent) to see if the D.9 clears. 4) Check visible wiring and plugs: with the boiler powered down, open the front flap and visually inspect plugs and cables to the PCB and sensor connectors for loose plugs, corroded pins or obvious damage. Do not touch connectors with the boiler powered.

More detailed diagnostic checks (only if you are competent and able to isolate power):

1) Confirm whether the fault is coming from an external analogue target signal or from the flow temperature sensor (NTC). On Vaillant boilers the service display often shows actual flow and return temperatures (check service readouts such as d.40/d.41). If flow temperature reading is missing, implausible, or constant, the flow NTC wiring or sensor may be suspect. 2) With power isolated, check that the plug for the flow NTC is fully seated in its socket on the PCB. Reseating a loose plug can sometimes clear the fault. 3) If you have a multimeter and feel comfortable using it: with power off and connectors disconnected, measure the resistance of the flow NTC sensor. Compare to the expected value in the boiler manual (many Vaillant NTCs are specified in the manual, often about 10 kΩ at 25 °C but you should confirm from your documentation). Check for open circuit (infinite) or short (near zero) which indicate a failed sensor or broken cable. Also check continuity of the sensor cable from sensor to PCB pins to identify a wiring break. 4) If the fault only appears when the external analogue regulator is connected, check the analogue input wiring for short/earth faults and verify the regulator is providing the correct type of signal per the boiler installer documentation. A mismatched or faulty external controller can force an incorrect flow target into the boiler.

What to do next / when to call a professional:

If the D.9 returns after a reset, if you find a faulty NTC (open or short), damaged cable, loose/damaged plug, or you cannot safely isolate and measure the signal, stop and call a Gas Safe registered engineer. Repairs that may be required include replacing the flow NTC sensor, repairing/replacing the wiring harness or connectors, repairing the external analogue regulator wiring, or in rare cases replacing or reprogramming PCB elements. Give the engineer the exact fault code, any other displayed codes, the results of your checks (reset attempts, whether disconnecting the external controller cleared the code, any visual cable issues, multimeter readings). This will speed diagnosis. Do not attempt gas or internal PCB repairs yourself.